Human-agent interaction (HAI), especially in the field of embodied conversational agents (ECA), is mainly construed as dyadic communication between a human user and a virtual agent. This is despite the fact that many application scenarios for future ECAs involve the presence of others. This paper critiques the view of an ‘isolated user’ and proposes a micro-sociological perspective on the participation roles in HAI. Two examples of an HAI in a public setting point out (1) the ways a variety of participants take part in the interaction, (2) how the construction of the participation roles influences the construction of the agent’s identity, and (3) how HAI, as a mediated interaction, is framed by an asymmetric participation framework. The paper concludes by suggesting various participation roles, which may inform development of ECAs.