This paper investigates mainland Chinese students' experiences of learning English as a second language (L2) in a multilingual university in Hong Kong, with particular attention to their negotiation of participation and identity. Drawing on a series of in-depth interviews with a group of mainland Chinese students, the study revealed that their participation in L2 practices appeared to be shaped by the contextual conditions in different communities within the university and mediated by their agentic responses to the contextual realities of these communities. While they gained fuller participation in L2-mediated activities in the local student community and the classroom community as a result of their appropriation of the associated language practices and their display of knowledge and competence in the related social practices, they struggled to gain access to L2 interactional opportunities in the community of exchange students, where their negotiation of positionalities was subject to the perceived unequal power relations between native and non-native speakers of the English. It is argued that their participation in L2-mediated activities in the university could be understood as a complex, dynamic, situated and co-constructed process of negotiating access, competence, membership, positionality, identities and discursive practices associated with different communities.