I use DNN for a couple of personal sites.  My employer has decided to build a portal for the industry we serve, and I think DNN is a good fit for most of it.  However, I have not seen how to support a key requirement - companies!  Essentially, we need to support the idea of a company as parent of users, with one or more users acting as an admin to add or remove other users from their firm (these orphaned users would still have access to the portal, but not data specific to that company.  They'll probably get hired by another company in this industry anyway, so will get hooked up to another company later. 

The database and forms development is not scary - just wary of messing with something as central as user authentication.  Any tips, warnings, or modules to recommend?  Thanks!