We propose a protocol that controls the members of a multicast group that send periodically status reports to all members. The protocol, called Multicast Access Protocol (MAP), limits the number of concurrent multicast reports as the group size becomes large. MAP is a decentralised protocol that provides an access control mechanism to an IP multicast group. The protocol supports members joining and leaving the group dynamically as well as changes in the underlying network topology. MAP is a self-configuring mechanism and requires every member to keep only local information independent of the group size. We describe the protocol both formally and informally.