A method is provided for inviting an assistant entity, such as a customer service representative acting through a endpoint system, into an existing network communication session established by a service system. The service system controls the joining of endpoint entities to the session and enables endpoint entities joined to the session to communicate with each other. The method involves a first party associated with an endpoint system already joined to the session, indicating to the service system that the presence of an assistant entity is required. The service system selects an appropriate assistant entity taking account of the current context of the communication session, and joins the selected assistant entity to the session.