Today, social activities are becoming increasingly popular and important to human life. As the number of contacts increases, however, the implicit social graph becomes increasingly complex, leading to a high cost on social activity organization and activity group formation. In this article, the authors present GroupMe, a group-aware smartphone sensing system that supports group management and activity organization in real-world applications. They first present a systematic methodology that can steer the development of mobile group awareness applications, then propose a multigranular group model. Based on the methodology and the model, the authors present their approaches that support various user needs on group formation and management, including closed group suggestion, open/opportunistic grouping, and new group member suggestion. Experimental results verify the effectiveness of the proposed approaches