The author proves an equivalence between large games with effective small groups of players and games generated by markets. Small groups are effective if all or almost all gains to collective activities can be achieved by groups bounded in size. A market is an exchange economy where all participants have concave, quasi-linear payoff functions. The market approximating a game is socially homogeneous--all participants have the same monotonic nondecreasing, and 1-homogeneous payoff function. Our results imply that any market (more generally, any economy with effective small groups) can be approximated by a socially homogeneous market.