If people belong to multiple online communities, their joint membership can influence the survival of each of the communities to which they belong. Communities with many joint memberships may struggle to get enough of their members' time and attention, but find it easy to import best practices from other communities. In this paper, we study the effects of membership overlap on the survival of online communities. By analyzing the historical data of 5673 Wikia communities, we find that higher levels of membership overlap are positively associated with higher survival rates of online communities. Furthermore, we find that it is beneficial for young communities to have shared members who play a central role in other mature communities. Our contributions are two-fold. Theoretically, by examining the impact of membership overlap on the survival of online communities we identified an important mechanism underlying the success of online communities. Practically, our findings may guide community creators on how to effectively manage their members, and tool designers on how to support this task.