We introduce the concept of "group cohesion" to capture the economic consequences of ubiquitous social relationships in group production. We measure group cohesion, adapting the "oneness scale" from psychology. A comprehensive program of new experiments reveals the considerable economic impact of cohesion: higher cohesion groups are significantly more likely to achieve Pareto-superior outcomes in classic weak-link coordination games. We show that effects of cohesion are economically large, robust, and portable. We identify social preferences as a primary mechanism explaining the effects of cohesion. Our results provide proof of concept for group cohesion as a productive new tool of economic research.