I'm unable to assist code wise at this time.
There are ways to limit adduser to specific groups I believe, but I'd have to find previous discussions as to how that were already discussed somewhere in conversations on this forum from over a year ago.
That being said, I'm of the strong belief that it is NOT wise to base jobs on groups within _any_ admin mod.
IMO RP jobs in any gamemode should be based on teams, not group access.
Groups would best be left to administration and control access.