public function addLDAPUserToGroup($userDn, $groupDn)
    {
        $members = $this->getLDAPGroupMembers($groupDn);

        // this user is already in the group, no need to do anything.
        if (in_array($userDn, $members)) {
            return;
        }

        $members[] = $userDn;

        try {
            $this->update($groupDn, ['member' => $members]);
        } catch (Exception $e) {
            throw new ValidationException('LDAP group membership add failure: ' . $e->getMessage());
        }
    }