Around the world, many people concerned with the state of the environment participate in environmental action groups. Much of their learning occurs informally, simply by participating in the everyday, ongoing collective life of the chosen group. Such settings provide unique opportunities for studying how people learn science in complex settings without being directly instructed. This study was designed to investigate learning and teaching that occurs through ordinary, everyday participation in environmental action. We draw on data collected during a 2-year ethnographic study of a coast-wide eelgrass-mapping project. Taking a whole activity as our unit of analysis, we articulate the forms of participation that volunteers take and theorize learning in terms of changing participation and expanding opportunities for action. The community-based eelgrass stewardship group we studied is both socially and materially heterogeneous, made up of people young and old and with different expertise. We show that changing forms of participation are emergent features of unfolding sociomaterial "inter"-action, not determinate roles or rules. Furthermore, the possibilities for learning expand when individuals have the opportunity to frame problems that arise in ongoing activity. In the setting of our study, attributions (dichotomies) such as "off-task/on-task" and "teacher/learner" are artificial. We suggest that by providing expanding opportunities, in the form of a variety of sociomaterial resources, science educators can rethink the design of school-based science learning environments.