Background: Many studies have pointed to the importance of trust and inclusiveness in teachers' collaborative groups. Here, we investigate how teachers negotiate dividing lines between "us" and "them" in moment-to-moment interaction. We also examine relationships between these negotiations, local arrangements, and teachers' opportunities to learn. Methods: We analyzed video recordings of two teacher groups, both oriented toward equitable mathematics instruction. We coordinated microinteractional analyses of four episodes with analyses of local arrangements that shaped interaction. Findings: In both groups, participants worked to construct a sense of solidarity as "us." Yet the threat of being positioned outside this "us"--as one of "them"--narrowed opportunities for teachers in both groups to engage in the vulnerable work of grappling with tensions and contradictions together. The salience of this threat was greater in one group than the other, however. We trace this to local arrangements that were differently designed, with different affordances for constructing "us" and "them." Contribution: The social negotiation of exclusion and belonging in teacher groups is highly consequential for teacher learning. Better supporting teacher learning is not just a matter of remediating individuals' skills or dispositions; it requires learning environments designed to support engagement with the tensions inherent in teaching.