WordPress at a glance
action is not described

register_new_user action-hook . WP 4.4.0

Fires after a new user registration has been recorded.


add_action( 'register_new_user', 'action_function_name_8658' );
function action_function_name_8658( $user_id ){
	// action...
ID of the newly registered user.

Where the hook is called

wp-includes/user.php 2432
do_action( 'register_new_user', $user_id );

Where the hook is used (in WP core)

wp-includes/default-filters.php 428
add_action( 'register_new_user', 'wp_send_new_user_notifications' );