Group formation is important in many economic contexts. The current literature on group formation assumes that individuals may join any existing group. In this paper, I consider the implications of social, geographic, and informational constraints to group membership decisions. I embed the players in a network of relationships, which constrains their choice of groups--they may only join a group if that group contains a member that they are connected to on the network. I then examine how this network constraint affects the equilibrium group structure. I show that even with complete information, unconstrained individuals form groups that are inefficiently large. When individuals are constrained, the resulting group structures are much closer to the socially optimal group structure, because the constraint limits the ability of the individual to free ride on the efforts of other group members. The efficiency of the outcome is related to the structure of the network constraint--outcomes are more efficient when networks are sparse and have few random connections.
