We consider the problem of providing communication protocol support for large-scale group collaboration systems for use in environments such as the Internet which are subject to packet loss, wide variations in end-to-end delays, and transient partitions. We identify a set of requirements that are critical for the design of such group collaboration systems. These include dynamic awareness notifications, reliable data delivery, and scalability to large numbers of users. We present a communication service, Corona, that attempts to meet these requirements. Corona supports two commttnication paradigms: the publish-subscn”be paradigm and the peer group paradigm. We present the interfaces provided by Corona to applications which are based on these paradigms. We describe the semantics of each interface method call and show how they can help meet the above requirements.