Large number of peer processes distributed in various types of networks are cooperating to achieve some objectives. We discuss a scalable group communication protocol named HCG (heterogeneous clock group) protocol. Messages are required to be causally delivered by using clocks in a group. An HCG group is composed of local subgroups in each of which processes are in a local or personal area network and which are interconnected in a wide-area network. In order to support the causally ordered delivery of messages in a scalable group, processes in local subgroups use physical and linear clocks while processes in a wide-area network adopt vector clock. We discuss how to causally deliver messages by using local synchronization mechanisms of each subgroup. We evaluate the HCG protocol in terms of number of messages ordered.