A communication system comprising: a first acquisition unit configured to acquire request information of session initiation and authentication information of session participation, the request information of session initiation requesting for initiation of a session between a first terminal and a second terminal and the request information of session initiation including whether or not there is session participation restriction for at least one terminal other than the first terminal and the second terminal, and the authentication information of session participation being provided for authenticating participation to the session when there is the session participation restriction; a session management unit configured to establish the session between the first terminal and the second terminal according to the request information of session initiation; a second acquisition unit configured to acquire request information of session participation and the authentication information of session participation being input by a third terminal, the request information of session participation being a request for participation in the session; and a participation determination unit configured to compare the authentication information of session participation obtained by the first acquisition unit with the authentication information of session participation obtained by the second acquisition unit when the request information of session participation is the request for participation in the session established by the request information of session initiation specifying the participation restriction, and the participation determination unit allowing the third terminal to participate in the session when the authentication information of session participation obtained by the first acquisition unit is identical to the authentication information of session participation obtained by the second acquisition unit, wherein the session management unit causes the third terminal to participate in the session when the participation determination unit allows the third terminal to participate in the session. a first acquisition unit configured to acquire request information of session initiation and authentication information of session participation, the request information of session initiation requesting for initiation of a session between a first terminal and a second terminal and the request information of session initiation including whether or not there is session participation restriction for at least one terminal other than the first terminal and the second terminal, and the authentication information of session participation being provided for authenticating participation to the session when there is the session participation restriction; a session management unit configured to establish the session between the first terminal and the second terminal according to the request information of session initiation; a second acquisition unit configured to acquire request information of session participation and the authentication information of session participation being input by a third terminal, the request information of session participation being a request for participation in the session; and a participation determination unit configured to compare the authentication information of session participation obtained by the first acquisition unit with the authentication information of session participation obtained by the second acquisition unit when the request information of session participation is the request for participation in the session established by the request information of session initiation specifying the participation restriction, and the participation determination unit allowing the third terminal to participate in the session when the authentication information of session participation obtained by the first acquisition unit is identical to the authentication information of session participation obtained by the second acquisition unit, wherein the session management unit causes the third terminal to participate in the session when the participation determination unit allows the third terminal to participate in the session.