A method and apparatus is provided for establishing a session upon satisfaction of particular conditions in a messaging service system such as SIMPLE Instant Messaging (IM), Push to talk over Cellular (PoC), and Converged IP Messaging) (CPM) services, in which a client provides particular conditions to a server separately for a predefined group session, an Ad-hoc group session and a one-to-one session, and the server establishes a session to the pertinent members upon satisfaction of the particular conditions. By doing so, a session reflecting particular conditions set by the client is established in the SIP-based system, thereby offering further differentiated messaging services.