Membership access for 3 different sub-directories

I just setup a multisite with 3 sub-directories.

How do I set up it up so that only the top level site is viewable for unregistered users?

So the users have to register at the top level site, and then by paying they will be able to view, and get access to either sub-directory site 1, 2 or 3, or a combination of them depending on the subscription they choose. I’m maybe thinking that they will first have to pay for viewing level 1 site, then upgrade to view level 2 site and then level 3 site. Top level site will only be for info about the different sub-directory sites.

Anyone know how this would be possible to do?