From what I understand, you want to be able to additionally define whether individual users inherit the privileges defined via their user group - is that right?
If this is what you want to be able to do, then unfortunately it is not how the feature is intended to be used.
The privileges that are defined to a user group, are intended to be inherited by EVERYONE in that group.
Consider it that the privileges assigned to that user group should be the absolute minimum privileges that are required by everyone in that group. If and when you then want specific users within that group to have additional privileges, you should then apply it on a per user basis.
If you are running into situations where you need to apply specific privileges to specific users, then I would suggest that you create a custom SQL query to select the targeted users. Then loop through the results and apply/remove the users privileges.
Using the SQL/loop method will save you having to do this on a per user basis - you just need the targeted users to all have something in common so you can select them.
