An operation method for an instant messenger server is provided. The method includes generating a schedule which depends on an account of a host in response to a schedule generation request of the host, determining invitees based on at least one of a chat room list and a friend list of the account of the host, synchronizing schedules which depend on accounts of users who participate in the schedule, based on whether at least one guest included in the invitees participates, and providing a notification of the schedule to at least one of the users who participate in the schedule, through a user interface (UI) of a chat room provided through the instant messaging service.