Please help – Multisite

I have couple questions.

So I set a multisite WP. for ex the main site is: http://www.location.com

A user registered to sub-site for ex: http://www.location.com/NYC
The user added to the global Network Users as well as the site he registered. Because it’s the first site he registered it will be the user’s primary site.

however, when the same user visiting in http://www.location.com/MIAMI
he was automatically registered/login and been added to the the site Users list.

I really don’t like that it’s auto joining the sites because site admin will have the data of someone from different location. I want that that users will be joining other sites only if they need.

So how can I do that users not going to automatically join other site in the network ?

Also how can a user who log in from the root site and redirect to the its Primary site they registered?
I mean there is any way to make it so they can choose which site they want to register from the main site ? any option for that ? You never heard about it ? this actually how every site work today..