I attend a couple of churches of Christ, both of which practice group prayer. If we are so concerned with serving God strictly with biblical authority (e.g. no music in worship, etc.) then surely there must be some explicit mention of group prayer in the New Testament. Could you please point this out to me?
I don't know what you mean by "group prayer" other than praying as a group. Technically prayers offered in worship are of that form. When Peter was in prison, many gathered to pray at Mary's house. "So, when he had considered this, he came to the house of Mary, the mother of John whose surname was Mark, where many were gathered together praying" (Acts 12:12). So praying as a group is not unheard of.