Individuals extracting common-pool resources in the field sometimes form outputsharing groups to avoid costs of crowding. In theory, if the right number of groups forms, Nash equilibrium aggregate effort should fall to the socially optimal level. Whether individuals manage to form the efficient number of groups and to invest within the chosen groups as theory predicts, however, has not been previously determined. We investigate these questions experimentally. We find that subjects do vote in most cases to divide themselves into the optimal number of output-sharing groups, and in addition do decrease the inefficiency significantly (by 50% to 71%). We did observe systematic departures from the theory when the group sizes are not predicted to induce socially optimal investment. Without exception these are in the direction of the socially optimal investment, confirming the tendency noted elsewhere in public goods experiments for subjects to be more “other-regarding” than purely selfish.