I like how you are applying the COI model to this scenario - it really helps to make sense of it. Have you any experience of this kind of thing from your own practice (or even as a student) that you can draw on and think about how you might do things differently?
Your comment about the negative comparison really compunds the problem for the group - perhaps it would have been better if the groups had worked in complete isolation? Did you or any of your group look at the other group work back in weeks 2 or 3? If so do you think it helped or hindered your groups' work?