I wanted new users to be automatically added to the main blog as contributors when they signed up as a User at the /wp-signup.php page.
I made the following change in the /wp-includes/wpmu-functions.php
function wpmu_create_user( $user_name, $password, $email) {
if ( username_exists($user_name) )
return false;// Check if the email address has been used already.
if ( email_exists($email) )
return false;$user_id = wp_create_user( $user_name, $password, $email );
$user = new WP_User($user_id);
// Newly created users have no roles or caps until they are added to a blog.
update_user_option($user_id, 'capabilities', '');
update_user_option($user_id, 'user_level', '');
Add this to the bottom of the above lines:
add_user_to_blog(1, $user_id, 'contributor');