public function getGroupMemberSet($principal)
    {
        $principal = $this->getPrincipalByPath($principal);
        if (!$principal) {
            throw new DAV\Exception('Principal not found');
        }
        $stmt = $this->pdo->prepare('SELECT principals.uri as uri FROM '.$this->groupMembersTableName.' AS groupmembers LEFT JOIN '.$this->tableName.' AS principals ON groupmembers.member_id = principals.id WHERE groupmembers.principal_id = ?');
        $stmt->execute([$principal['id']]);

        $result = [];
        while ($row = $stmt->fetch(\PDO::FETCH_ASSOC)) {
            $result[] = $row['uri'];
        }

        return $result;
    }