In this paper, a new radio communication mechanism, called MACS, is proposed. It supports a group communication facility in an environment of autonomous mobile robots, i.e. a particular open and distributed dynamic computer network. Since many computers may participate in group communication in such environments, the bounds of the communication bandwidth becomes a serious bottle neck when the number of members of a communication group is not limited. Furthermore, the open system means the dynamics of the network topology, so group membership should also be determined dynamically following the location or conditions of each robot. MACS introduces a new slot assignment scheme, called TDMA/TP, that applies TDMA to temporal and partial areas of the network dynamically, on demand. Since this protocol never occupies the communication channel, multiple group communications can exist concurrently.