This is the last time this group has been talked about. We have covered much of the information previously, although most of the information about them comes from Revelation 7. What they do in Revelation 14.
The scene where there is a vast multitude before the throne will come up again. Who they are, what they are, and how they got there can be worked out.
For a start, they come from all tribes, peoples, and languages. 
We are told that the number is myriads of myriads. A myriad is ten thousand. Daniel states plainly, Ten thousand, ten thousand. One hundred million. Not only is the gospel preached to every tribe, but someone is saved from every tribe. Some good news in a book about bad news. Rev 7-13 Then one of the elders addressed me, saying, “Who are these, clothed in white robes, and from where have they come?” I said to him, “Sir, you know.” And he said to me, “These are the ones coming out of the great tribulation”. They have washed their robes and made them white in the blood of the Lamb. 
Where have they come from? It says out of the tribulation, which is the last three and a half years. They are the first resurrection, and in the last verse, it is the same description as for those whose names are found in the book of life at the second resurrection.