Somehow for it. I do really want a more complex system. But you'd need some more info in each group:
group <group_id> {
name: <name>
rights: <key>{,<key>{,...}}
inherit: <group_id>{,<group_id>{,...}}
}
What I think of is, that group_id really is a number (the old GM level) and name is what is displayed as his "rank". All rights the group has will be after rights, like:
rights: can_drop,can_bypass_statmax
etc.
I would also go along with SkzBR's suggestion.
Like this we can create redundant groups that are just patterns to inherit for other groups.