Research-based instructional strategies in physics promote active participation in collaborative activities as a primary means for students to construct understanding. This emphasis is in line with situated learning theory, in which learning is indicated by a student's increasing centrality in a community. In both perspectives, to learn more is to engage more centrally: to start discussions, conduct experiments, write on the board, decide when a question has been answered, and so on. In a study of small-group collaborative learning activities in introductory physics classrooms at three different universities, we observe that as students engage with one another and with instructors, they are not only negotiating physics concepts, but also negotiating social positioning. Some students are centered (and their contributions are valued), while others are marginalized (and their contributions are neglected). The aim of this research is to become conscious of how centering and marginalization shape the way physics is taught and learned.