Bones is the person who "initiates" Tony into the gang, wrestling with him briefly. Tony throws Bones - a boy much larger than Tony - and wins the respect of Bones.
This occurs at the church early in the novel. Bones, in this way, becomes Tony's first friend in the novel. The rest of the gang welcomes Tony in as a member of the group. Later they remain friends at school as well, banding together when they feel that there they may not all be accepted by the other children at school.