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Large number of peer processes are cooperating in peer-to-peer systems. In this paper, we discuss a hierarchical group protocol aiming at reducing communication and computation overheads for a scalable group of peer processes. A hierarchical group is composed of subgroups each of which is furthermore composed of subgroups. Even if messages are causally ordered in one subgroup, the messages may not be required to be causally ordered in a whole group. We discuss how to globally causally order messages by ordering mechanisms in each subgroup.