I won't argue that point. Believe it or not though, the ones that don't pull their wait still get something out of it. The students that do the majority of the work will learn even more. Include a self evaluation and a group evaluation at the end of the project. You'll be surprised at how honest the evaluations will be even from the dead loads. If any of the work is done in class you'll know quickly who knows what they are doing and who doesn't.
I also seem to remember that three people was found to be the optimal group size.