It's quite clear that Paul - talking to those in Rome, the church, the believers.
those who are in Christ Jesus v1
so that the requirement of the Law might be fulfilled in us v4
However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you v9
So then, brethren v12
fellow heirs with Christ v17
revealing of the sons of God. v19
but also WE ourselves
So it's not the Jews (though some might be there), but it is the brethren which naturally includes Apostles, teachers, elders etc - that which makes up the brethren and 'sons of God'.
The firstfruits is the spirit deposit given to believers. This works in them until their change at Christ's return. It is a symbol of the 'First fruit', Jesus, who dwells in us with the Father through the spirit. (2 Corinthians 1:21-22, Eph 1:14)