I know you can filter Participants lists by groups or by many filters--def. 3.8--(no role! I use facetoface which allows you to manually add people to sessions who are not enrolled but doesn't give them a course role, so they're just hanging out in course limbo--filter by no role, resolve all of them at once!) but I don't think you can do that for grades or for course completion reports. Which is unfortunate...it would be super nice.
Restrict access is great for having one group see a specific set of activities and the other a completely different set...and then completion of either set can open the whole group activity.