Efficient coordination in large groups is a fundamental issue in economic organizations. We consider the weak‐link game with the feature of economies of scale that is necessary for voluntary group growth, in a minimal setup with exit and merger options. We show that large groups with efficient outcomes prevail in most communities. Seed groups form that seem to set a norm of both maximal effort level and mutual trust that there will be no panic when seeing noisy dips. Initial doubters are eventually converted into norm abiders. A restart further speeds up the process of coordination success.