1
You’re forgetting the user reference. So if I interpret you properly, you want something like –
groupless = detailedassignment.course.subscribed.exclude(user__username__in = group_list.filter(assignment=detailedassignment).members.objects.values_list('user__username'))
OR try this –
groupless = UserProfile.objects.filter(courses_list__assignment = detailedassignment).exclude(group_list__assignment=detailedassignment)
Also as an aside, your model design looks somewhat skewed, with same references running across each other. And there’s no need to add backward references (like in model `Course). Django gives you the functionality for backward referencing. See https://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward
0
So, when the backward reference have been removed, I have used a related_name in my ManyToManyField and Rahul’s answer with a little edit :
groupless = User.objects.filter(course_list__assignment=detailedassignment).exclude(group_list__assignment=detailedassignment)
If it can be useful.. 
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