Here at Federal University of Brasil, we are developing an enroll plugin to automatic create users and courses and enroll/unenroll user in courses. Once we have the "enroll" field in role_assignments, we just do a left join with our table and know wich ones was included by our script and need to be removed.
Now we have to enrol/unenrol user in/from groups, but we don't have that "enrol" field to identify which users were enrolled by our script and which ones was manual.
So, i suggest (and can upload a trivial patch) to add the field "enrol VARCHAR(20) DEFAULT 'manual' " to the "groups_members" table and add a parameter to groups_add_member() function to specify the enrollment plugin.