Group Work is often a difficult grade to give, as informal observation by the teacher is one of the only ways to determine if students are working well together and all pitching in to the task at hand.
Well, I've found a way to make grading student participation in a group easier for everyone! Each member of a group is responsible for not only grading every other person in their group, but they also grade themselves. By using a rubric divided up into relevant categories specific to collaboration and individual student effort, the teacher is able to get a clearer picture of the contribution each student gave during the duration of the group project. I pass the rubric out at the beginning of the group project/activity so all students are aware that their peers will be evaluating them and that they will be expected to evaluate their peers based on their effort and participation.
When the teacher looks at the group member evaluations and records the scores, usually a pattern prevails regarding whom (if anyone) contributed the most, the least, or if they seemed to put in equal effort. I oftentimes notice that students will grade themselves lower than their peers. Other times, they are worried their peers will be mad if they don't evaluate them by giving them the maximum amount of points. To prevent anxiety, I let them know that I will be the only one who will look at the evaluations, and I have them use carrels/offices to fill out the evaluations for the other students.
I hope you find this resource valuable in your classroom!