Hello, I was wondering how it would be done to allow multiple networks under one big social site.

I'm trying to figure out the best way to have one social site where the users don't join but have join through a network. a network would be a company. So the owner of the company would sign up and they would have a company blog page/ users and what not.. but for the users to sign up they have to sign up through a company. The users have a profile and can do private messaging. but don't have a blog.