This study explores the process of Community of Practice (CoP) building for language teachers' professional development through the support of a WeChat group. WeChat is an instant messenger app that provides a multimodal platform for one-on-one and group interactions through text, audio and video. In order to support the implementation of flipped classrooms in a Chinese programme offered in an Australian university, a teachers' chat group was created on WeChat for just-in-time and collaborative professional development. This qualitative study focuses on three dimensions in the CoP, namely, mutual engagement, a joint enterprise, and a shared repertoire to examine how these Chinese teachers worked together to form their own CoP in their WeChat group. Qualitative data were collected through group chat interactions on WeChat, teacher reflective journals and a focus group discussion through audio conferencing on WeChat. Discussions of the strategies and challenges involved in the building of an active and sustainable online CoP through social media contribute to our understanding of professional development in a digital age.