I am building a multisite that allows users to create their

I am building a multisite that allows users to create their own blog (something like edublogs.com). However, I also want those sites to allow users to register to their own site without the need to go through the main site. I am using domain mapping and pro sites, but can’t find a way to make this happen. I don’t want users that own their own site on my network to feel like they are within a network. I want their users to be able to register to their site only as if it was a independent WordPress install. How can this be made possible? thanks