A selection of intellectual goods produced by online communities - e.g. open source software or knowledge bases like Wikipedia - are in daily use by a broad audience, and thus their quality impacts the public at large. Yet, it is still unclear what contributes to the effectiveness of such online peer production systems: what conditions or social processes help them deliver quality products. Specifically, while co-contribution (i.e. bipartite networks) are often investigated in online collaboration, the role of interpersonal communication in coordination of online peer-production is much less investigated. To address this gap we have reconstructed networks of personal communication (direct messaging) between Wikipedia editors gathered in so called Wikiprojects - teams of contributors who focus on articles within specific topical areas. We found that effective projects exchange larger volume of direct messages and that their communication structure allows for complex coordination: for sharing of information locally through selective ties, and at the same time globally across the whole group. To verify how these network measures relate to the subjective perception of importance of group communication we conducted semi-structured interviews with members of selected projects. Our interviewees used direct communication for providing feedback, for maintaining close relations and for tapping on the social capital of the Wikipedia community. Our results underscore the importance of communication structure in online collaboration: online peer production communities rely on interpersonal communication to coordinate their work and to maintain high levels of engagement. Design of platforms for such communities should allow for ample group level communication as well as for one-on-one interactions.