add_rewrite_rule() to route to file other than index.php

add_action('parse_query', function(&$wp_query) {

    if (array_key_exists('iid', $wp_query->query_vars) && $wp_query->query['pagename'] === 'invite') {

        $iid = get_query_var('iid');

        // Do some stuff

        exit();

    }

    return;

});