To get to the Group Reports, navigate to Reports in the navigation tab. The reports menu will open to the side, select the 'Group Reports' option.

This report lets you see a series of quick stats about your user groups - Groups, Users Commenced, Average Mark, and Average Time Spent - before drilling down into individual groups and providing the same stats for each of them. You can click on an individual group name to see what users are enrolled in that group, and specific statistics for each course that group is taking.