Multicasting is increasingly used as an efficient communication mechanism for group-oriented applications in the Internet. In order to offer secrecy for multicast applications, the traffic encryption key has to be changed whenever a user joins or leaves the system. Such a change has to be communicated to all the current users. The bandwidth used for such rekeying operation could be high when the group size is large. The proposed solutions to cope with this limitation, commonly called 1 affects n phenomenon, consist of organizing group members into subgroups that use independent traffic encryption keys. This kind of solutions introduce a new challenge which is the requirement of decrypting and reencrypting multicast messages whenever they pass from one subgroup to another. This is a serious drawback for applications that require real-time communication such as video-conferencing. In order to avoid the systematic decryption / reencryption of messages, we propose in this paper an adaptive solution which structures group members into clusters according to the application requirements in term of synchronization and the membership change behavior in the secure session. Simulation results show that our solution is efficient and typically adaptive compared to other schemes.