Social instant messaging services are emerging as a transformative form with which people connect, communicate with friends in their daily life — they catalyze the formation of social groups, and they bring people stronger sense of community and connection. How-ever, research community still knows little about the formation and evolution of groups in the context of social messaging — their life-cycles, the change in their underlying structures over time, and the diffusion processes by which they develop new members. In this paper, we analyze the daily usage logs from WeChat group messaging platform — the largest standalone messaging communication service in China — with the goal of understanding the processes by which social messaging groups come together, grow new members, and evolve over time. Specifically, we dis-cover a strong dichotomy among groups in terms of their lifecycle,