Fellowship and friendly social relations during free time, referred to here as leisure-based sociability, is a prominent reward of participation in many groups based on volunteer membership, consisting for this review mainly of amateurs, hobbyists, altruistically oriented volunteers, and the associations of these three. This benefit is analyzed according to two subtypes: sociable nonprofit associations and social clubs. The goal of this issue of the Voluntaristics Review is to examine the leisure component of these two subtypes as framed in the serious leisure perspective (SLP) as set out in Stebbins (2007/2015; Stebbins, in press, see also www.seriousleisure.net), put nonprofit sociability in organizational context, and then review the empirical literature bearing on it. Studies and theoretic treatises approaching nonprofit groups from another angle (eg, organizational structure, management issues, funding sources, governmental regulation, type of employment) are not be reviewed. Specifically, this review centers on the relevant books, articles, and chapters listed in the SLP website, which centers on amateurs, hobbyists, and career volunteers (the serious pursuits), casual leisure, and project-based interests and includes its extensions in the theory and research on the leisure-related aspects of aging and retirement, arts and science administration, library and information science, positive psychology, therapeutic recreation and disability studies, and tourism and event analysis. Compared with the various specialties in leisure studies, the SLP casts by far the broadest theoretical and empirical net in that interdisciplinary field. The research reviewed shows that such talk – generically known as socializing -- reflects one or more of 14 themes. In general, members find sociability in these clubs and associations in and around the core activities they pursue there and on which the two subtypes have formed. The studies reviewed here, taken together, provide considerable validation of the proposition that leisure-based sociability is a prominent reward of participation evident in a multitude of volunteer groups. Leisure-based sociability is itself micro-analytic in scope, but viewed through the lens of the SLP, it can be further understood using meso- and macro levels of analysis.