Applying an ecological and evolutionary perspective, this study examines the evolution and the sustainability of “mixed-mode groups,” a type of voluntary association created and organized online to interact physically in geographically defined ways. Meetup.com is a website that facilitates the creation and coordination of mixed-mode groups. Analysis of interviews with 34 Meetup group organizers and a longitudinal analysis of 100 randomly selected Meetup groups revealed the evolutionary processes at the group and population level, respectively. Specifically, the ecological factor of population density, the demographic factor of group age, the group's profit orientation, experience of leadership change and shared leadership, and external ties played decisive roles in predicting group survival. Implications of the findings for theoretical and practical contributions are discussed.