How to register to a subsite without registering to the root site ?

In my multisite I have 4 subsites all of them with buddypress installed.

When a user registered to subsite with Facebook he automatically register and login to the subsite he registered for.
THAT'S AWESOME !! because the users primary site is the subsite they first registered.

However, when registering to a subsite through the register form or signup.php it's registering the user to the root site + the subsite he just registered, giving primary the primary_url to the ROOT site ! I don't want it. I would like to have the same behavior like in Facebook registration! I don't want users to register automatically to my root site when they register to the subsite. I actually don't mind as long they the Primary_url will be belong to the subsite the first register.

How it can be done ? PLEASE ! ! !

More details:
In another situation, if i'm using Multisite User Management plugin. I changed my subsites to have other roles. A user register to one of the subsite. The plugin register the user automatically to all of the other site I set with the plugin. Also the user primary_url is the latest subsite I was created in my network.

Also Join my Mulitiste plugin is not effect anything here.

I'm just trying to learn the behavior of the registration process on this thread you can see what I'm looking for in the facebook registration:
https://premium.wpmudev.org/forums/topic/redirect-log-in-user-to-their-primary-site#post-744329

What I'm looking for is simple - User registered to subsite2 through subsite2/register. Subsite2 will be the user PRIMARY SITE.
So when a user login through the ROOT site I redirect them to where they registered first.

Please let me know if this process can be done in other way perhaps ?