Share users across multisite but not permissions


I'm looking for a solution to share users across several websites, I mean when you're registered on one site you're registered on all others.

I used

define('CUSTOM_USER_TABLE', 'wp_users');
define('CUSTOM_USER_META_TABLE', 'wp_usermeta');

So I got the login/password informations shared across websites which is good.

However I got a problem with the user role as it is prefixed with db_prefix.

How can I automatically assign a default user role (let's say "contributor" if the user successfully connect and as no current user role?