This article addresses performance assessments that involve collaboration among students. We apply the Hawkes process to infer whether the actions of one student are associated with increased probability of further actions by his/her partner(s) in the near future. This leads to an intuitive notion of engagement among collaborators, and we consider a model-based index that can be used to quantify this notion. The approach is illustrated using a simulation-based task designed for science education, in which pairs of collaborators interact using online chat. We also consider the empirical relationship between chat engagement and task performance, finding that less engaged collaborators were less likely to revise their responses after being given an opportunity to share their work with their partner.