public function setAdmin(string $admin = "F") : CNabuDataObject
    {
        if ($admin === null) {
            throw new ENabuCoreException(
                    ENabuCoreException::ERROR_NULL_VALUE_NOT_ALLOWED_IN,
                    array("\$admin")
            );
        }
        $this->setValue('nb_user_group_member_admin', $admin);
        
        return $this;
    }