The circumcised believers are simply Christian believers who have been circumcised, those who come from a Jewish background.1 At this point in time, it includes the vast majority of the church in Judea. There is no indication in the Greek text that anything like a circumcision group or party was in existence at this point (see Galatians 2:12; Acts 15:5).2
2 So when Peter went up to Jerusalem, the circumcision party criticized him, saying,