The phrase mennskir menn, "human men," is sufficiently clear to convey who is meant. Mennskir menn are distinct from náir, dead men, corpses. These are not the human men of Midgard, but living human beings residing in Hel. According to our sources, there are only two living men found in the underworld: Lif and Lifthrasir hidden in Hoard-Mimir’s grove. Hel is otherwise the home of the dead.

That these mennskir menn are not the Æsir, as Snorri concluded, is made clear by Sigrdrifumál 18, the only other place in Eddic poetry where the phrase is used. There mennskir menn are listed alongside the three classes of divine beings, indicating that they are distinct from them. Each have runes:

In Egil’s Saga ch. 25, the term is used to distinguish human men from giants (en líkari eru þeir þursum að vexti og að sýn en mennskum mönnum).

In Gylfaginning 54,  Snorri informs us that Lif and Lifthrasir survive the conflagration of Ragnarök hidden in Hoddmimis holt.  He doesn’t tell us when they arrived there, but does cite the source of his information, Vafþrúðnismál 45:

A variant of this verse, which occurs in the Codex Upsalensis manuscript of Snorri’s Edda, ends with the line: ok þar vm alldr alaz, they resided “there through the ages.”

We are told that the new world will be inhabited by righteous people, lead by the blameless Baldur. It will be a new golden age.  Considering the conditions on earth at the time of Ragnarök, it is unlikely that any innocent human beings were left. Völuspá R44/H37 informs us that:

Thus it would make sense if the gods had hidden these two mortals away, long before the commencement of Ragnarök, when innocent mortals untouched by corruption could still be found in the world.  

Vafþrúðnismál 45 informs us that Lif and Lifthrasir were hidden in Hoddmimis holt, “Hoard-Mimir’s grove” [also Mimis holldi, “Mimir’s flesh” in a variant of this verse found in Snorri’s Edda].  Gylfaginning 15 informs us that Mimir's well is located "where Ginnungagap once was," in one of the oldest regions of the universe. It stands at the very center of the cosmos, directly below the trunk of the world-tree. Thus, Yggdrasil is called Mimameiðr, Mimir's Tree. In Grímnismál 27, Hoard-Mimir’s grove is referred to as the "hoard of the gods" (hodd goða). There Mimir collects the divine treasures, preserving them and keeping them safe until Ragnarök has passed. Hoddmimis holt is the archetypical sacred grove, located at the center of creation, where life first arose. Here Ymir was sacrificed and the upper worlds created. Once Surt's flames consume the upper worlds, Hoddmimis holt is the source from which life will emerge once again. It is the womb of the world. Baldur and Höðr, the twin sons of the All-father and the Earth Mother are located here, waiting to be reborn.

Mimir is an ancient figure, whose origin extends far back into Indo-European times. We find cognates of him in both the Rigveda and the Avesta, where he is called Yama and Yima, respectively.

In the Indian Rigveda, the lower world is the kingdom of death, of which Yama is king (Rigv., X. 16, 9; cp. I. 35, 6, etc). It is a glorious country with inexhaustible fountains and imperishable light. Like Mimir, Yama dwells under a tree "with broad leaves." He gathers the ancient fathers there and drinks with the gods (Rigv., X. 135,1).

 “In the Tree clothed with goodly leaves where Yama drinks with the Gods,
The Father, Master of the house, tends with love our ancient Sires.”

In the ancient Iranian sources, Yama’s counterpart Yima is a holy and mighty ancient being, counted among the oldest seers and prophets of antiquity. A hymn of sacrifice, dedicated to the sacred mead, (homa, the soma and soma-madhu of Rigveda and mjoð of the Edda), says that Yima and his father were the first to prepare the mead of inspiration. In his kingdom, there was neither cold nor heat, neither frost nor drought, neither aging nor death. A father by the side of his son resembled a fifteen year old youth. No evil could cross into Yima's kingdom. 

Yasna, ch. 9, verse 5 (L.H. Mills translation): 

 "In the reign of Yima, swift of motion, was there neither cold nor heat, there was neither age nor death, nor envy demon-made. Like fifteen yearlings walked the two forth, son and father, in their stature and their form, so long as Yima, son of Vivanghvant ruled, he of the many herds!"

  In regard to the origin and purpose of the kingdom ruled by Yima, Vendidad Fargard 2, I (from The Zend-Avesta part 1, translated by James Darmesteter, 1887) tells the following:

   21. The Maker Ahura Mazda, of high renown …called together a meeting of the celestial gods…          To that meeting came the fair Yima, the good shepherd of high renown …he came together with the best of the mortals.

  22.  And Ahura Mazda spake unto Yima, saying: 'O fair Yima, Upon the material world the fatal winters are about to fall, that shall bring the fierce, foul frost; upon the material world the fatal winters are going to fall, that shall make snow-flakes fall thick, even an aredvi deep on the highest tops of mountains. 

  23. From three places, beasts should be driven to well-enclosed shelters; those that live in the wilderness, and those that live on the tops of the mountains, and those that live in the bosom of the dale, under the shelter of stables.

   24. Before the winter, this land had meadows. Before that time the water (the rain) was wont to flow over it, and the snow to melt; and there was found, in the material world, water-soaked places, in which were visible the footprints of the cattle and their offspring.

   25. Therefore make thee an enclosure, long as a riding-ground on every side of the square, and thither bring the seeds of sheep and oxen, of men, of dogs, of birds, and of blazing red fires. Therefore, make thee an enclosure, long as a riding-ground on every side of the square, to be an abode for man; an enclosure, long as a riding-ground on every side of the square, for oxen and sheep.

   26. There thou shalt make waters flow in a bed a hathra long; there thou shalt settle birds, on the green that never fades, with food that never fails. There thou shalt establish dwelling-places, consisting of a house with a balcony, a courtyard, and as gallery.

  27. Thither thou shalt bring the seeds of men and women, of the greatest, best, and finest on this earth; thither thou shalt bring the seeds of every kind of cattle, of the greatest, best, and finest on this earth.

  28. Thither thou shalt bring the seed of every kind of Tree, of the greatest, best, and most fragrant on this earth. Thither thou shalt bring the seed of every kind of fruit, the best tasting and the most fragrant on this earth. All those seeds shalt thou bring, two of every kind, to be kept inexhaustible there, so long as those men shall stay in the enclosure.

  29. There shall be no humpbacked, none bulged forward there; no impotent, no lunatic; no poverty, no lying; no meanness, no jealousy; no one with decayed tooth, no leprous to be confined, nor any of the brands wherewith Angra Mainyu stamps the bodies of mortals.

 Like Hoard-Mimir’s grove, Yima's garden has been established to endure a terrible winter upon the earth. It was planned to preserve the best and the fairest in the world of living beings. The location of the garden is not stated, but as the surface of the earth is devastated by the unparalleled winter, we can scarcely conceive the garden as situated in the upper world. That it is subterranean is expressly stated in Bundahishn, ch. 29, 14, where it is located under the mountain Yimakan; and that it was originally subterranean follows from the fact that Yima of the ancient Iranian records is identical with Rigveda's Yama, whose domain and the scene of whose activities is the lower world, the kingdom of death.

 As Yima's enclosed garden was established on account of the great winter, which occurred in time's morning, it continues to exist after the close of the winter, and preserves through all historical ages the treasure of uncorrupted men, animals, and plants which were collected there in the beginning of time. The purpose of this is mentioned in Menog-i khard, (Chap. 27, 24-31). There it is said that after the conflagration of the world, and in the beginning of the regeneration, the garden which Yima made shall open its gate, and from there men, animals, and plants shall once more fill the devastated earth (E. W. West translation, Sacred Books of the East, Vol. 24, 1880):

   28. …it is declared in revelation that mankind and the other creatures and creations of Ohrmazd, the Lord, are mostly those which shall perish.

  29. One shall afterwards open the gate of that enclosure formed by Yim,

  30. And the people and cattle, and other creatures and creations of the creator Ohrmazd, shall come out from that enclosure,

  31. And arrange the world again.

 Thus, the “hoard of the gods” is a biological treasure-trove, hidden in the underworld. When the old earth is burnt and sinks into the sea, the new world rises anew. Without a new act of creation, the world teems with life.

There, on the ‘new earth’, we find Baldur and Hödr, who previously resided in Hel. There too, we find Lif and Lifthrasir hidden in Hoard-Mimir’s grove; and Hoenir, the god given with Mimir as a hostage to the Vanir during the Van-As war. Mimir’s grove is thus the Germanic reflection of Yima’s garden.
 
No doubt, Lif and Lifthraisr, the mennskir menn who were hidden in Hoddmimis holt have been there “through the ages”— the axe age, the sword age  when shields are cloven, the wind age, the wolf age, before the earth sinks. These must be the people that Odin, riding Sleipnir into Hel before Baldur’s death,  sees within “Hel’s high hall” who anxiously await Baldur’s arrival.

They are called ásmegir, “sons of the Æsir. ”The word used here to describe their mood is ofvæni, which according to the Egilsson Lexicon means “stærk forventning spænding” (“strong expectation”, “excitement”) from væna “to hope.” The LaFarge/Tucker Glossary to the Poetic Edda translates the term as “intolerable expectation or suspense”. The ásmegir appear to eagerly await Baldur’s death.

This has puzzled scholars who interpret the ásmegir as the Æsir themselves, so much so that some have proposed missing lines:
 
Ursula Dronke, Poetic Edda II, p. 158:
 
“Two preceding lines may have been lost here (cf. the ten-line stanza 11). Sijmons-Gering suggest that the lost lines probably noted the happy anticipation of the inhabitants of Hel, in contrast to the Æsir’s fearful anxiety, at the thought of Baldr’s death.”
 
The more common approach is simply to soften the translation. Carolyne Larrington, for example, translates this as “despair”. Thus, the “sons of the Æsir,” the gods themselves, were in despair. However, if we interpret the ásmegir as the inhabitants of Mimir’s grove in the lower world, as the poet probably intended, then their excitement is understandable. They are eager to welcome Baldr into their company and have poured out mead in anticipation of his arrival. What is a misfortune for the world, is a boon to them. This may explain the description of Baldr’s hall Briedablik in Grímnismál 12; there the poet says “ the fewest evil runes lie.” It is the well-fortified grove at the center of the universe, in which nothing corrupt may enter.
 
When Hermod arrives to the same hall on Sleipnir, in Gylfaginning 49, which seems to be derived from a poetic source, he must leap a high wall that only Odin’s horse can vault. Odin himself, riding Sleipnir, regularly visits this place to consult Mimir’s head. Most likely, this is the same place seen by Hadding on his visit to the underworld with a supernatural female guide in Book 1 of Saxo’s Danish History. There, Saxo says: 

“The nether gods purposed that he should pay a visit in the flesh to the regions whither he must go when he died. So they first pierced through a certain dark misty cloud, and then advancing along a path that was worn away with long thoroughfaring, they beheld certain men wearing rich robes, and nobles clad in purple; these passed, they at last approached sunny regions which produced the herbs the woman had brought away.”
 
“… Then a wall hard to approach and to climb blocked their further advance. The woman tried to leap it, but in vain, being unable to do so even with her slender wrinkled body; then she wrung off the head of a cock which she chanced to be taking down with her, and flung it beyond the barrier of the walls; and forthwith the bird came to life again, and testified by a loud crow to recovery of its breathing.”

 
Despite its location in the land of the dead, it is populated with living creatures; nothing can die there. A rooster’s head, thrown over the wall, returns to its body and crows aloud, testifying to life.  Like Yima’s garden, Hoard-Mimir’s grove is an oasis of life surrounded by the kingdom of death. In later sources, such as the Icelandic Fornaldarsögur, it is referred to as the realm of the giant Gudmund of Glæsisvellir (‘The Glittering Plains’). His land is known as Grund (cp. Jörmungrund), and as Óðainsakr, “the acre of the not-dead.” Gudmund is the brother of the wicked giant Geirrod, slain by Thor. They dwell in neighboring kingdoms. Gudmund is a beneficent ruler who treats strangers hospitably, offerinfg them abundant food and drink produced in his private gardens.  Gudmund of Glæisvellir is most likely a historical reflection of Mimir, the ruler of the sacred grove at the center of the cosmos. One thing is certain, this legendary figure, who appears in several sources, has no Christian precedents. He thus must have been heathen in origin. Among heathen figures, Mimir alone provides a close parallel.