3-tier user site with large file repository?

I am looking at a new project that involves a 3 tier user base and a large file repository.

Basically, it's Admin -> Domain -> User

1. The admin has 6 levels of access and grants access to a domain (or list of domains) when they register (off-site for now).
2. The domain contact then informs all members of the domain they can register at the site.
3. The user registers and is tied for the life of their account to the Domain.
4. The user can only see certain content and most importantly only download certain files.

Has anyone ever seen a system like this, or better yet, know a good place to start to write a system like this? I'm open to using something like PunBB since I've used it elsewhere but I'm not sure if customizing it would be more trouble than it's worth since PunBB and most everything else is based on just users and users groups and I have this extra level hiding in there.

Any thoughts or suggestions on using PunBB as a base for this (or thoughts in general are welcome too!)

Thanks,
Andrew