Many modern computing environments involve dynamic peer groups. Distributed simulation, multi-user games, conferencing applications and replicated servers are just a few examples. Given the openness of today's networks, communication among peers (group members) must be secure and, at the same time, efficient. This paper studies the problem of authenticated key agreement in dynamic peer groups with the emphasis on efficient and provably secure key authentication, key confirmation and integrity. It begins by considering 2-party authenticated key agreement and extends the results to Group Diffie-Hellman key agreement. In the process, some new security properties (unique to groups) are encountered and discussed.