(see also the group dynamics section in the Concepts and theories chapter)
Collaborative learning is about collaborating to learn, but also about learning to collaborate (Collazos et al., 2002 - ref below) and for this reason attempts are sometimes made to assess participants' progress in terms of the quality of the learning process, not just the outcome.
Perhaps the least interesting form this takes is assessing relative contribution to the process, sometimes to encourage greater participation by individuals in the group. As Janet Macdonald (2003) puts it: "Even the more reluctant students can be encouraged to engage in online tasks or structured debate if their participation is part of an impending assignment." [Macdonald, J. (2003). Assessing online collaborative learning: process and product. Computers & Education, 40(4), 377-391.]
A more constructive reason for assessing process (also mentioned by Macdonald) is to encourage students to reflect on and improve their collaboration skills.
Collazos, C.A., Guerrero, L.A., Pino, J.A., & Ochoa, S.F. (2002). Evaluating collaborative learning processes. In J.M. Haake and J.A. Pino (Eds.), CRIWG 2002, LCNS 2440, 203-221.