Revelation 7:1-8 describes the 144,000, then verse 9 begins with "after these things." This is simply a time marker in John's vision, not in prophetic time. It means afterward, later, John saw an innumerable multitude. The Greek does not say that the events of Revelation 7:9-17 immediately follow or that they are part of the preceding information—only that John received this information after the previous information. Perhaps it could follow right after, but the Greek does not require it.
John says "no one could number" this multitude (verse 9). Why? Notice that this multitude is comprised "of all nations, tribes, peoples and tongues." That would seem to be a great many people! The context indicates a large number, not just an indeterminate one.
John sees these people "standing before the throne and before the Lamb"—not with Him on the throne ruling, but before the throne in judgment. Remember, judgment occurs over a period of time. The firstfruits have already been judged and have risen at Christ's return, so this multitude has to be people in a different group who are judged later.
Revelation 3:21, written directly to Laodicea, says God grants overcomers the reward of sitting with Him on His throne! Thus, they have qualified to be in the first resurrection, having been judged to be worthy now (I Peter 4:17). We have already seen that whether we die in Christ or are still alive, we are "changed—in a moment, in the twinkling of an eye, at the last trumpet" (I Corinthians 15:51-52) as firstfruits. None of those in the first resurrection will stand "before the throne" for judgment when He returns, for we are currently under judgment, which God will complete and reward us at His Son's return (Revelation 11:18).
This multitude, then, cannot be in the first resurrection! In the process of judgment, they have donned white robes, a growth in spirituality that takes considerable time.
The Innumerable Multitude