<p>Digital community archives and many digital humanities projects serve as critical infrastructure for community-based cultural knowledge, but they struggle with sustainability. Prior work has illuminated numerous sustainability factors for digital cultural knowledge infrastructures that are developed and maintained by communities, but there is relatively little empirical work on the roles of communities themselves. Based on a comparative multi-case study of four projects, we offer a conceptual framework—the Oyster Model—for understanding community engagement as a sustainability factor in community-based, digital cultural knowledge infrastructures. Sustainable infrastructures identify and engage a range of stakeholder groups—teams, partners, contributors, users, and allies—in different roles and adapt to dynamism and change among these groups. This model characterizes reciprocity as a facet of community-centered approaches to sustainability, to refine understanding of the mutually beneficial relationship between digital knowledge infrastructures and the groups that create and maintain them. This work aims to bridge a gap between the community archives literature and relevant work on the sustainability of digital humanities scholarship.</p>