When a quiz is set to Separate groups and is restricted to some groups, the users belonging to the group(s) that are not allowed to take the quiz should not be listed in the "enrolled users who have not attempted the quiz" list.
Also, when those group(s) are selected in the Separate groups drop-down list, the "Full regrade" and "Dry run a full regrade" buttons/links should not be displayed, since they are meaningless in this context.
I suggest that, in this use case, the Separate groups drop-down list should not display those groups that are not allowed to take the quiz or, better, display them greyed out (and un-selectable, of course).
The same use case applies to the Questionnaire plugin "View all Responses" display.
What do quiz users think?