Open source software projects such as Apache and Mozilla present an opportunity for information visualization. Since these projects typically require collaboration between developers located far apart, the amount of electronic communication between them is large. Our goal is to apply information visualization techniques to assist software engineering scientists and project managers with analyzing the data. We present a visualization technique that provides an intuitive, time-series, interactive summary view of the social groups that form, evolve and vanish during the entire lifetime of the project. This visualization helps software engineering researchers understand the organization, structure, and evolution of the communication and collaboration activities of a large, complex software project.