Automatically add a registered user on the wordpress multisite network even to just one other site

The following code works for me:

add_action( 'wpmu_activate_user', 'myplugin_registration_save', 10, 1 );
function myplugin_registration_save( $user_id ) {
    $blog_id = 13;
    add_user_to_blog($blog_id, $user_id, "subscriber");
}