These are ones who will serve with Christ in his kingdom as kings and priests. It can also be extended to all those who side with Christ.
9 They sang a new song, saying,
“You are worthy to take the book
and to open its seals:
for you were killed,
and bought us for God with your blood
out of every tribe, language, people, and nation,
10 and made us kings and priests to our God,
and we will reign on the earth.”
14 For as many as are led by the Spirit of God, these are children of God. 15 For you didn’t receive the spirit of bondage again to fear, but you received the Spirit of adoption, by whom we cry, “Abba! Father!”
16 The Spirit himself testifies with our spirit that we are children of God; 17 and if children, then heirs: heirs of God and joint heirs with Christ, if indeed we suffer with him, that we may also be glorified with him.
Notice in Romans 8 that even Christ is a joint heir and that there’s only one Father.