This paper aims at contributing to the debate in Mathematics Education about the understanding of the dynamics of students' group interactions by proposing an interpretative lens, which defines four modes of participating in a group on the basis of different kinds of utterances, gestures, postures, and glances that each student makes. We apply this lens to two selected cases of students working in a small group, and, by comparing and contrasting similarities and differences observed through our interpretative lens, we attempt to understand how, and under which circumstances, the students reach mathematical understanding as a group, or not.