(power is over-rated)
For groups to work well, the members must buy in to the importance of succeeding as a team rather than the importance of protecting their individual reputations (or in the case of an assignment, their individual marks). Protecting the individual means keeping a second set of books to be able to cut the ties with other members if the going gets tough. Groups that are not wholeheartedly committed to success as a group often end up resorting to backstabbing, seeking to be evaluated individually, or seeking to have teammates penalized.
It is tempting to say that in a course, the group has no authority over the teammates therefore it is artificial compared to the work world where people can be fired. Being able to fire someone gives you positional power. In fact, at work, most group work is with people over whom you will have no more authority than you have over classmates. You must collaborate on the basis of personal power, not on the basis of positional power. Even with your own subordinates, if you don’t have the personal power to make them want to achieve your goals, if you are getting things done only by using positional power (the ability to fire them), you are very close to the door. (Guess which door.)