code: 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;
      }
docs: Build group.
  
  @param mixed $group
  
  @return array