This paper discusses the design and implementation of Simmcast, an object-oriented framework for network simulation with specific support for group communication. The design of the framework is focused towards simplicity and extensibility. The aim is to allow a spectrum of experiments ranging from evaluation of abstract group communication models to simulation of more detailed multicast protocol behavior. Simmcast employs a process-based discrete-event model on which building blocks are combined and extended in order to create new simulation environments. Network parameters are given in terms of numeric distributions (fixed or probabilistic), which can be replaced without recompilation. This extensive use of numeric distributions combined to the extendible framework structure allow an abstract experiment to evolve into a detailed one by progressively increasing the level of detail and sources of non-determinism of the constituting blocks.