You need to make the functions available, they are located in wp-includes/user.php. So load the user.php via require e.g. like this:
require( ABSPATH . WPINC . '/user.php' );
But keep in mind, if you need to do that and load wp-load.php manually, then you likely have not done it right. Meaning, you should aim for a solution that is integrated into the WordPress process, so that no manual stuff like that is necessary.