Ensemble is a Group Communication System built at Cornell and the Hebrew universities. It allows processes to create process groups within which scalable reliable fifo-ordered multicast and point-to-point communication are supported. The system also supports other communication properties, such as causal and total multicast ordering, flow control, etc. This paper describes the security protocols and infrastructure of Ensemble. Applications using Ensemble with the extensions described here benefit from strong security properties. Under the assumption that trusted processes will not be corrupted, all communication is secured from tampering by outsiders. Our work extends previous work performed in the Horus system (Ensemble's predecessor) by adding support for multiple partitions, efficient rekeying, and application defined security policies. Unlike Horus, which used its own security infrastructure with non-standard key distribution and timing services, Ensemble's security mechanism is based on off-the shelf authentication systems, such as PGP and Kerberos. We extend previous results on group rekeying, with a novel protocol that makes use of diamond-like data structures. Our Diamond protocol allows the removal of untrusted members within milliseconds.