private function buildGroup($group)
    {
        if (is_null($group)) {
            $group = [
                'filter' => [
                    'is_to_all' => true,
                ],
            ];
        } elseif (is_array($group)) {
            $group = [
                'touser' => $group,
            ];
        } else {
            $group = [
                'filter' => [
                    'is_to_all' => false,
                    'group_id' => $group,
                ],
            ];
        }

        return $group;
    }