Secure multicast has a variety of applications in e-commerce, e-banking, command and control, video-on-demand, and other internet-based services. In this paper, we present algorithms to improve on the number of re-keying messages (overhead) needed to add and delete sets of users to a secure multicast group. We present upper and lower bounds on the number of re-keying bits to add or delete groups of users from secure multicast groups. We also show how to amortize the cost of group re-keying over time to avoid long periods of system overhead that can potentially block the transmission of desired data at times when large numbers of users simultaneously join or leave multicast sessions.