Note: For this to work, you must have created the Groups you want to check as a Group Set.
Follow these steps to reach the screen where the groups and their members are all listed:
1) Click on 'Tools' from the Course Menu.
2) Click on 'Groups' from the list of tools. This will take you to a page that lists all of the groups available in the course.
3) In the upper right corner of the screen you are taken to click on 'Group Sets'. This will change the list to show the Set that the individual groups make up.
4) Click on the Action Link for the Group Set that is of interest and select 'Edit Set Enrollments'. This will take you to a page that lists all of the Groups in the Set. From here, the students enrolled in the different groups are found in the 'Selected Items' box. All of the students that are listed in the 'Items to Select' box are not currently enrolled in a Group. From this screen you can also manually enroll students into, or unenroll students from, any given Group.