Disclosed is a method for enabling a receiving-side Push-To-Talk (PTT)-over-Cellular (PoC) user to acquire ad-hoc group invitee information and selectively establishing a PoC session according to preference when a pre-established session and an auto-answer mode have been set between a receiving-side PoC user equipment and a PoC server in a PoC system. Particularly, in order to efficiently utilize the receiving-side Session Initiation Protocol (SIP) session setting, disclosed are a solution for transferring information related to other users expected to participate in group communication through user-plane signaling, and in addition, access rules, service setting, timers and a method for changing a response set between the client and server in order to support the solution.