manage users in multisite like edublogs or wp.com

Hi

I have a great missunderstood about users in multisite.

If i join edublogs or wp.com and i create a blog there i dont have access to other blogs created in those sites, isn’t it?

However i was testing all the morning with that in my multisite instalation, and when i register in one subsite, it redirects me to the main site(networksite) so if i register i belongs to the network and with this credentials i have access to other subsites.

I dont want that?

An example with citys in USA.

Imaging i create a site oferring massage in different cities site.com/newyork and site.com/texas and a client from texas enter in site.com/texas in order to be registered, then is redirected site.com/sign-up.php and in his inbox arrive a username and a password, and i have realised that with these username and password is able to access to site.com/newyork.

How to avoid that?

Please help, i am becoming crazy with that.