Software development has traditionally been regarded as an activity that can only be effectively conducted and managed within a firm setting. However, contrary to such assertions, the open source software development (OSSD) approach, in which software developers in Internet-based communities coordinate to voluntarily contribute programming code, has recently emerged as a promising alternative to developing high quality software. Although many high profile cases of successful OSSD projects exist (e.g., Apache, OpenOffice, Emacs, PHP), the harsh reality is that the vast majority of OSS projects fail to take off and become abandoned. A commonly cited reason for the failure of OSS projects is the lack of developers in the project teams, or put differently, the inability of the software project to bring together a critical mass of developers. In this paper, we examine how OSSD project teams are formed. More specifically, we investigate whether prior collaborative ties impact OSSD team formation and developers’ joining behaviors. Using software project data from real world OSSD projects, we empirically test the impact of previous collaborative ties on software team formation. Overall, we find that the existence and the amount of prior collaborative relations in the developer network do increase the probability that an OSS project will attract more developers and that a developer’s prior relationships with a project initiator do increase the likelihood that a developer will join a project initiated by a past collaborator. We also explore the performance implications of early team formation behaviors. We discuss the implications of our results with respect to open source software development and software project management.