User signup for WPMU subsites ?

I have setup a WPMU installation at the main site:

and several subsites (via subdomains + domain mapping) e.g:

When a new user signs up at the subsite he gets only a user id at the main site, and can't acess the subsite he originally signed up as a user :slight_frown:

How can I setup that when a user signs up at a subsite he will get a user at the subsite (or subsite + mainsite) ?

I have tried everything, but no chance...