In many applied settings, a group of agents decide whether to participate in an activity that requires a critical mass of participants to be worthwhile. This property can give rise to multiple equilibria. Existing equilibrium selection results typically rely on strong payoff assumptions. We replace these with a weak single crossing property and show that the predictions have a common form: an agent plays a best response to some fictional distribution of the participation rate of her opponents. An application to equity crowdfunding is studied.