nthesis bv thinks this is interesting:

Because users group membership is defined in two different locations, it can be difficult to find out which groups exactly a user is a member of. A convenient command to check this is groupmems. Use, for example, the command groupmems -g sales -l to see which users are a member of the group sales. This shows users who are a member of this group as a secondary group assignment, but also users who are a member of this group as the primary group assignment.