I use cooperative often in my ELD classes. Though my system is a constant work in progress, I do have a few approaches that work for me. One is that though students have a group project (and a group grade from it), they also have individual components of the project that I grade them on. I also monitor each group and each person in it. I use a clipboard with their names listed and actually spend time talking to not only the group but each person in it to see what they are doing. When all is said and done, I also have them evaluate their work as a group and in this evaluation they have to give a grade for the group and for EACH person in it (remember that I verify this with my own checklist of who was working and to what degree). Last but not least is that when each group presents its work to the entire class, their class members give a grade with a detailed explanation behind it. I average all the grades (including my own) and THAT is the final grade:)
In the past I have created rubrics for students to follow as they are working on group projects. This way they are learning about what they need to do and practicing what they need to do before they are assessed. The other option would be to have students themselves create the rubric on cooperative learning. That way they own the process and would likely do much better when it comes time to assess.