Just about everyone in education is talking about how working in groups is a 21st Century skill. It is my contention that working in groups is a sub-optimal strategy for teaching. I can only offer my immediate anecdotal thoughts.
If there are two students working a group, they share the benefits but not the cost. There's a tendency for one student to do all of the work because the benefit is shared. So one student will free ride or shirk.
I found this Harvard paper on the topic. But more on this topic will follow as I put this into a game theory context.