Communities of practice (CoPs) are important sites of social interaction and are of growing concern in international relations. Much attention has been devoted to examining the existence and effect of these communities, and the kinds of practices and identities they coalesce around. Relatively less attention has been afforded to the relations between a CoP and what lies outside of it. In this article we examine encounters at the boundaries of CoPs. Empirically, we examine the boundaries and interactions of a community of diplomatic practice in the context of the Association of Southeast Asian Nations (ASEAN) as regional officials engage with extra-regional diplomats and “dialogue partners” operating within and alongside the organization. We find that the “boundary work” of these interactions are important sites of social interaction that constitute communities in important ways and make possible meaningful changes in identity and behavior. More broadly, we show that ASEAN’s unique position as a hub of a multilateral architecture that extends to a wider state membership provides prime terrain to inquire about what happens in liminal spaces of encounter between social agents of varied communities. To advance our claims, we draw on sixty-one semi-structured interviews with regional and extra-regional officials in Jakarta and elsewhere in the region, alongside analysis of public statements from diplomatic practitioners.