In the Control Panel, from the Main Menu select 'Static Data' and then ' Manage Sub-Group Members'.
To see a list of everyone assigned to a specific Sub-Group, select that Sub-Group from the dropdown list and then click on the second red button - 'Show Members of This Sub-Group'.
To remove a specific user from a Sub-Group, click on the email address in the list and then confirm that you want to remove them. This user is now no longer assigned to this Sub-Group.
Selecting the Sub-Group from the dropdown menu and clicking the second red button again ('Show Members of This Sub-Group') will confirm that they have been added.