A topology of groups: What GitHub can tell us about online collaboration

•Hierarchical, star-shaped graphs that are completely centered on one maintainer.•Complex network structures that show reciprocity and clustering•Hierarchical, star-shaped graphs with several maintainers•Hierarchical graphs with a brokerage user that often decompose into two subgroups.•Very modular graphs that decompose into many individuals and subgroups. In this work, we study the collaboration patterns of open source software projects on GitHub by analyzing the pull request submissions and acceptances of repositories. We develop a group typology based on the structural properties of the corresponding directed graphs, and analyze how the topology is connected to the repositorys collective identity, hierarchy, productivity, popularity, resilience and stability. These analyses indicate significant differences between group types and thereby provide valuable insights on how to effectively organize collaborative software development. Identifying the mechanisms that underlie self-organized collaboration on digital platforms is important not just to better understand open source software development but also all other decentralized and digital work environments, a setting widely regarded as a key feature of the future work place.