I have to register an user only in the present blog. After I visit http://multisite.com/subblog/wp-login.php?action=register it redirects to http://multisite.com/wp-signup.php. Basically register an user here i quickly have acces to any or all blogs and I'm not going this.

How do i solve this?

Thanks

Have you got Wordpress Multisite setup? If that's the case, from the experience I believe you need to login towards the primary user interface because the superadmin. Next, towards the top of the sidebar within the user interface there must be a network configurations button.

Click that, and after that you need to have the ability to access a listing of blogs within the install, and after that you are able to choose a person blog and add customers after that.

The bottom line is by using a multiuser setup you have access to all blog configurations in the one superuser account which can be why you are getting the redirect problem.