Group chats serve as dynamic spaces where digital collectiveness transforms individual interactions into collective action. This chapter explores the evolution of group chats from early digital communication tools to essential community-building platforms in the platform era. It examines how celebrities leverage group chats to engage fans, foster loyalty, and mobilize communities for creative, social, and philanthropic endeavors. The chapter also delves into the role of moderators in maintaining inclusivity, enforcing community guidelines, and navigating power dynamics within group chats. Through real-world case studies, such as the BTS ARMY and the Weibo fan groups, it highlights how these spaces enable collective action and strengthen the bonds between celebrities and their audiences. Finally, the chapter offers practical insights for everyday users seeking to build meaningful, inclusive digital communities