- If NB_GID is set, it is likely that files created should have NB_GID
  as their gid (if it doesn't matter, then this change is irrelevant.
  If it does matter, the liklihood that 100 matches with what the user
  wants is low).
- When NB_GID is set, create a new group with that gid.  Then, set
  this as the primary gid of the user.  Set the user's supplemental
  gids to NB_GID,100.