As a result of recent changes to U.S. undergraduate mathematics instruction, graduate student teaching assistants (TAs) may now be asked to teach in ways that they did not themselves experience as students. One such example is the use of collaborative group learning. The preparation TAs receive to teach in these new ways has the potential to shape instructional experiences for a substantial number of undergraduates now and in the future as TAs become mathematics faculty. This paper describes how professional development activities evolved in response to TAs' needs. These TAs taught classes where students spend significant time working mathematical problems in small collaborative groups. As more was learned about challenges TAs faced, activities were designed and revised. The activities were designed to promote reflection on issues of teaching and learning and included viewing class videotapes and observing collaborative groups for extended periods of time.