When students are working in groups, it is difficult to know everything that goes on or how to assess each person in order to hold them accountable. I like to have each member rate their teammates as well as themselves. Then I add up how many points each person gets from all the teammates and divide to get a participation grade. I always let them know what their group mates will be grading them on before beginning the project or assignment.
A student might receive from him/herself and group mates:
For a total of 52 out of a possible 60 (total of 15 possible from each rater). This person would receive an 87% for participation.