First of all I search for my answer using search form but I didn't find anything useful and correct for what I'm looking for.
I would like to know if it is possible (easily) to do something that able users to register to a particular blog.
When you activate "user can register" the link made in the login-form is a link to "wp-login.php&action=register".
this redirection when "is_multisite()" is true redirect to "signup.php".
In this file wp check if the site is the main site and if it isn't it redirect to the signup page of the main site.
It is possible to let signup.php on the main site working to create users and blog but having in the same time the ability to register to one blog like in not multisite mode.
I hope i'm understanding cause i'm not very good to writing in English ^^