The present invention provides a communication system, a control apparatus, a program and a recording medium that controls participation in a session using presence information on participation member and information of the session itself. A communication system in which a plurality of communication terminals are connected to each other through a network to establish communication between the communication terminals has a presence information registering section that registers presence information of the communication terminals, a participation condition setting section that sets a participation condition for participating in the communication, a communication information managing section that manages communication information related to a state of the communication, and a communication control section that controls participation in and separation from the communication of the communication terminals.