In secure group communications, users of a group share a common group key. A key server sends the group key to authorized new users as well as performs group rekeying for group users whenever the key changes. In this paper, we investigate scalability issues of reliable group rekeying, and provide a performance analysis of our group key management system (called keygem) based upon the use of key trees. Instead of rekeying after each join or leave, we use periodic batch rekeying to improve scalability and alleviate out-ofsync problems among rekey messages as well as between rekey and data messages. Our analyses show that batch rekeying can achieve large performance gains. We then investigate reliable multicast of rekey messages using proactive FEC. We observe that rekey transport has an eventual reliability and a soft real-time requirement, and that the rekey workload has a sparseness property, that is, each group user only needs to receive a small fraction of the packets that carry a rekey message sent by the key server. We also investigate tradeoffs between server and receiver bandwidth requirements versus group rekey interval, and show how to determine the maximum number of group users a key server can support.