We introduce the concept of "group cohesion" to study the economic consequences of social relationships in team production. We measure group cohesion, adapting the "oneness scale" from psychology to group level. A series of experiments, including a pre-registered replication, reveals that higher cohesion groups are more likely to achieve Pareto-superior outcomes in weaklink coordination games. Judged against benchmarks, the effects of cohesion are economically large. We identify beliefs rather than social preferences as a primary mechanism explaining the effects of cohesion. Our comprehensive evidence establishes group cohesion as a powerful production factor and a useful new tool of economic research.