John B
I have heard and read some of this army in Joel, and some professing Christians believe they are to be this army.

 From my understanding of this army according to scripture; "This army is great and terrible" it is to be in "That Day" which is the day of wrath of the Lord. This army comes up from the pit, if you will.
In Zephaniah 2:1 we read Gather yourselves together, yea, gather together, O nation [not] desired.( Esau)
 In verse 3)  [A warning] goes forth to the Meek of the earth to seek righteousness, those who have wrought his judgment, that they [may] be [hid] in the day of the Lord.

( Which the hidden ones are the believers who are come through the open door of Rev 3:7,8)

 BUT these that are mentioned in verse 9)  are in fact the seed of Esau. We can also [know] this because in verse 9  it refers to " those who say they are Jews and are not."

I have gone to great lengths in study concerning Esau.
Rome: in the days before, and during the days of our Lord, considered the Edomites to be [ a kind of Jew] and these were set to rule over Israel as the Herod's and even the religious leaders of that time.

Why? did Jesus have such contempt for them?

Zephaniah 1:15 That day is a day of wrath, a day of trouble and distress, a day of wasteness and desolation, a day of [darkness and gloominess,] a day of clouds and thick darkness, 16) A day [of the trumpet and alarm] AGAINST the fenced cities, and AGAINST the high towers

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And I will bring distress upon [men,] that they shall walk like blind men, because they have sinned against the Lord: and their blood shall be poured out as dust, and their flesh as the dung. Neither their silver nor their gold shall be able to deliver them in [the day] of the Lord's wrath; but the whole land shall be devoured by the fire of his jealousy: for he shall make even a speedy [riddance of all them that dwell in the land.]

The dry bones of Ezekiel 37 I read that they are the restoration of the nation of Israel and that they will be placed back in their land. TODAY they are.

Ezekiel 37:21-28  But; they have [not] given up their idols and "DAVID" is not king over them, which tells us this is [future yet]
Why?
1 Corinthians 6:3 KJV
Know ye not that we shall judge angels? how much more things that pertain to this life?
This is during the 1000 year reign of Christ. We will be righteous judges over areas of this earth, and DAVID will sit over Israel [ seed of David] Jesus Christ.

An example of this is in the Exodus with Moses taking the advice of his father-in-law. Some were set over tens, some over 100s, and some 1000s, and anything that these could not handle were sent [then] to Moses.

 The same will be sent to Christ [in that day.]
I know this is latter days [also] because the Lord through Ezekiel uses the words [for ever.]

I have not studied [out] Joel's army and do not feel I can give you an astute  answer. It does seem though that Esau's seed is the target.

It was due to a study concerning the 'coats of skins" God made Adam and his wife Eve that I saw there [also] that Esau was; [as a seed of Abraham,] while Abraham was still yet Abram, connected Esau to [the coats of skins] through the killing of Nimrod.

This was a vision of Nimrod that a seed of Abram would kill him.
I used the book of Jasher' for part of that study, only because the Bible gives us [two] direct mentions of Jasher.
Joshua 10:13 KJV
2 Samuel 1:18  KJV
In the New testament there are named two men " Jan'-nes and Jam'-bres"
2 Timothy 3:8 KJV
Now [as] Jan'-nes and Jam'-bres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith.

If we had not read "Jasher" we would not know and understand who these two men were, because they are [not] mentioned by name in the Exodus story.

Their father is Ba' laam.
By this study did I learn that Ham was [a thief] in the story of Noah cursing Canaan. Genesis 9:20-29 KJV
Te'man is mentioned only 11 times in the Bible.

btw: Zepho son of Eliphez, son of Esau was the [first] king of Rome.

Blessings
KML