Disclosed are a method and a system in which a client intending to participate in a chat Push-to-talk over Cellular (PoC) group, presents a session participation-invitation reservation to a server when the number limit has been exceeded pursuant to the policy of the chat PoC group, or when a chat PoC group ID exists but a corresponding session has not been initiated. Herein, when a client intending to participate in a chat PoC group receives a response indicating participation is not possible because the number limit of the chat PoC group has been exceeded, the client performs invitation reservation until participation is possible.