A social activity planning method includes the following operation. A database storing multiple candidate invitees with their geographical locations and interest scores towards a topic, and a social graph including vertices representing the candidate invitees and edges connecting two vertices representing mutual acquaintance of the associated candidate invitees is accessed. A social activity request including an invitation number, information about a social activity, and a familiarity condition for the social activity request is received. At least one feasible solution including the invitation number of the candidate invitees and meeting the familiarity condition is generated. A group of selected invitees are found based on the feasible solution and the information about the social activity.