Mobile fitness apps have gained popularity as a way of improving healthy behaviors. This study investigates the design and influence of fitness apps from a novel “group” perspective, focusing on the effects of group-level social interaction features in exercise participation. Through empirical studies on a unique dataset, we propose and verify a conceptual framework that shows that (1) group role model inspiration and group comparison features facilitate the intra-group assimilation and inter-group contrast processes and further significantly improve group participation, and that (2) offline activities moderate the effects of group comparison features on group participation. This study contributes to the literature on online communities and fitness apps/service design by demonstrating the motivating power of group-level interactions and online–offline engagement. It also provides practical guidelines for fitness app developers.