How to limit registration to only one site of a multisite network

I am setting up a sports broadcasting Multisite network. The main site will aggregate content from subsites ("child sites"?) and display it. There will be a subsite for each state, then subsites for each school under each of those states.

So I need to have a registration process that allows users to navigate to their state's site, for example dsnamerica.com/idaho/ then go to the registration page on that site (dsnamerica.com/idaho/registration) and after registering, signing up for a site, and activating their account, they should only be able to post content to that new site they created (dsnamerica.com/idaho/idahofallshighschool/).

1) Is this already how a Multisite registration works? Or will they have access to every blog including the main one by default?

2) If not, how do I set it up like this?

3) THANK YOU

ps. The links above don't actually go to what I say they go to. I'm setting it all up now. Just example links...