During adolescence, peer processes play a significant role in development. Whereas peer relations have been studied extensively in the school context, far less research has examined peer processes that occur within extracurricular and out-of-school time activities. This study used focus groups to explore the role of peer processes in shaping developmental processes and outcomes across four distinct types of out-of-school activities in Santiago, Chile. Across all extracurricular activities, three common features were identified: voluntary participation, symmetrical relationships, and a sense of belonging that establishes clear boundaries bewtween ingroup and outgroup. These themes were integrated into an overall model of peer processes that illustrates how voluntary participation and shared activity results in a peer collective identity based on shared interests. These peer processes resemble a developmental arc bridging collective and individual development, fostering socioemotional development, identity work, and positive relationships and trust within a set of symmetrical relationships. The results illustrate the ways in which peer processes may support development within out-of-school activities.