The virtual synchrony model for group communication has proven to be a powerful paradigm for building distributed applications. In applications that require the use of a large number of groups, significant performance gains can be attained if these groups share the resources required to provide virtual synchrony. A service that maps user groups onto instances of a virtually synchronous implementation is called a Light-Weight Group Service. This paper discusses the usage of Light-Weight Group protocols in dynamic environments, where mappings cannot be defined a priori and may change over time. We show that it is possible to establish mappings that promote sharing and, at the same time, minimize interference. These mappings can be established in an automated manner, using heuristics applied locally at each node. Experiments using an implementation in the Horus system show that significant performance improvements can be achieved with this approach.