Determining contributors' expertise, role, and individual importance are fundamental for assessing their impact on a software project. Currently-used expertise metrics are agnostic to contributor roles and can lead to incorrect characterizations. To address these issues, we operationalize contributor expertise and role. First, we revisit current expertise metrics and show that their use bundles many different aspects, creating ambiguity. Second, we introduce clearly-defined contributor roles, which capture multiple project facets. Third, we propose a graph model, based on contributor collaborations, that captures the hierarchical structure of the contributor community in a concise yet informative way. We demonstrate the model's usefulness in two ways: (a) for identifying the structure and evolution of contributor interactions, (b) for predicting contributor roles. We substantiate our study using two large open-source projects, Fire fox and Eclipse. Our systematic approach clarifies and isolates contributor role and expertise, and sheds light onto the dynamics of contributors within software projects.