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I don't see it as a task so easy someone could tell you how to do it in a sentence or two - it really sounds like a major project to me. I mean, first you have to create a bunch of usergroups, then set promotions, and only then start coding to show (or sort by?) usergroups.

Keep searching for mods though - I know you aren't the first to want to do something like this, so there may be something out there already that will work that I'm just unaware of.