There are all different arrangements, Im sure. Murder Inc was made up of killers who were not official inducted members. They were hired or contracted to do hits. Even 'boss of bosses' Maranzano hired Irish killer "Mad Dog" Coll to kill Luciano. He arrived just a few minutes too late and Maranzano was killed before Coll arrived to kill Luciano or the entire course of mob history would have been totally different.