As online discussion boards become a popular medium for collaborative problem solving, we would like to understand patterns of group interactions that lead to collaborative learning and better performance. In this paper, we present an approach for assessing collaboration in online discussion, by profiling student-group participation. We use a modularity function to compute optimal discussion group partitions and then examine usage patterns with respect to high-versus low-participating students, and high- versus lowperforming students as measured by grades. We apply the profiling technique to a discussion board of an undergraduate computer science course with three semesters of discussion data, comprising 142 users and 1620 messages. Several patterns are identified, and in particular, we show that high achievers tend to act as ‘bridges’, engaging in more diverse discussions with a wider group of peers.