Regular volunteers are instrumental to the functioning of numerous nonprofit organizations. To effectively engage and retain these valuable volunteers, nonprofit leaders need to cultivate cohesion among them. To address this challenge, our study employed multi-level analyses to investigate the dynamics of social and task cohesion among regular volunteers in Flemish nonprofit sports clubs (N = 557, nested within 52 sports clubs). Additionally, the study delved into the motivating styles leaders can rely on to foster social and task cohesion. The findings uncovered that regular volunteers constitute “true groups” within nonprofit organizations, engaging in interpersonal interactions that affect their collective sense of harmony and unity around shared goals. Furthermore, the results showed that group cohesion is contingent upon leaders cultivating an autonomy-supportive climate wherein volunteers feel comfortable expressing their thoughts and ideas. Nonprofit leaders can use these insights to develop cohesive groups within their organization.