How can I access user information on a subdomain?

Found two solutions:

1)

global $current_user;
$wp_get_current_user_id = $current_user->ID;

$wp_get_current_user_id = apply_filters( 'determine_current_user', false );

Which then in helloworld.php would be return:

echo json_encode( array('wordpress_user_id' => wp_get_current_user_id) );