Reliable group multicasts provide a nice abstraction for communicating data reliably among group members and have been used for a variety of applications. In this paper we present Corona, a group communication service for building collaboration tools and reliable data dissemination services in Web-based environments, where clients connect independently of other clients and are not necessarily connected to the group multicast services all the time. The key features of Corona are: (1) the shared state of a group consists of a set of objects shared collectively among group members; (2) Corona supports multiple state transfer policies to accommodate clients with different needs and resources; (3) the communication service provides the current group state or state updates to new clients even when other clients are not available; (4) the service supports persistent groups that tolerate client failures and leaves. We show that the overhead incurred by the multicast service in managing each group's shared state has little impact on the latency seen by the clients or the server throughput. We also show that the multicast service does not have to be aware of the client-specific semantics of the objects in the group's state.