Large number of peer processes are cooperating by exchanging messages in autonomic, peer-to-peer systems. In this paper, we discuss a hierarchical group protocol aiming at reducing communication and computation overheads for a group of large number of processes. A hierarchical group is hierarchically composed of subgroups. Each subgroup supports its own mechanism to causally order messages. Even if messages are causally ordered in one subgroup, the messages may not be causally ordered in another subgroup. We discuss how to globally causally order messages by using local mechanisms to causally order messages in subgroups. In addition, we evaluate the protocol.