One of the biggest reservations that teachers have about assigning group work revolves around the issue of grading. How does one do it fairly? What if one or two members of a 5-person team end up doing most of the work?
One answer is to allow the students a say in the grading process. Knowing before the project even starts that they will be asked to fill out a self-evaluation and a peer evaluation form at the end of the project, and being aware that that evaluation will be factored into the final grade can be a huge motivating factor for keeping all students engaged and working productively.
Here is an example of such a form. I take each person’s scores (if they are in a 5 person group, there will be 5 scores per person), add them together, and average them. This final average becomes their “group participation” score on the rubric that I use for grading the project. How heavily you weight this aspect of the project is up to you.