This is exactly what I found as the forum I help with moved from vB to XF. You REALLY SHOULD completely re-think the way you view permissions. "We've always done it like this" isn't sufficient.
So, to re-iterate the above.
You should have ALL your members set up in a UserGroup with a base, common set of permissions.
Then each category of increased membership is placed in a secondary usergroup which has ONLY those extra permissions this group is entitled to.
If your structure is incremental (i.e as members get "promoted") then add them to yet another secondary group with ONLY the next set of permissions. And so on.
With the base level of permission (things everyone can do) at the bottom you can run these things as linear hierarchies (in "series" if you will) or in parallel - or a combination of both.
It's very flexible and actually very easy to manipulate - but first you have to forget your previous ideas and structures completely.