Add them how? They can't register? You can't add them via the admincp? We need more details.
Meanwhile, make sure you are using all default files (no default vbulletin files should show up in the list from maintenance > diagnostics > suspect file version) and disable your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php
PHP Code:
define('DISABLE_HOOKS', true);