Group communications have been studied in the wired Internet for many years and remain a very hot research topic, especially on extending the existing achievements into mobile and wireless network environment. This paper identifies some interesting problems in mobile group communications regarding dynamic group membership due to member joining and leaving, dynamic locations due to node mobility, and dynamic networks due to node/link failures. These problems are solved by proposing a ring-based hierarchy of proxies with bi-directional tokens. The proposed hierarchy is a combination of logical rings and logical trees, which takes advantage of the simplicity of logical rings and the scalability of logical trees. More importantly, such a combination makes the proposed protocol based on this hierarchy more reliable than the existing tree-based protocols. Theoretical analysis and simulation studies of the proposed protocol show that: (1) It scales very well when the size of a group becomes large; (2) It is strongly resilient to failures that occur in the network. It is particularly suitable for those service providers and network operators which have deployed their machines in a hierarchical setting, where each machine can be locally configured to know the information about its sibling and parent machines.