A communication system including: a first acquiring unit configured to obtain start request information for specifying whether participation in the session of a terminal is limited or not, and participation authentication information for authenticating participation in the session for the terminal; a session manager configured to establish the session; a second acquiring unit configured to obtain participation request information for requesting participation of a third terminal in the session, and the participation authentication information input from the third terminal; and a participation determination unit configured to compere the participation authentication obtained by the first acquiring unit and by the second acquiring unit, and determine whether participation of the third terminal in the session is allowed or not, and the participation determination unit configured to record terminal information of the third terminal, and to reject a subsequent participation request of the third terminal in the session thereafter.