code: public Observable<GroupResult> groupAsync(List<UUID> faceIds) {
          return groupWithServiceResponseAsync(faceIds).map(new Func1<ServiceResponse<GroupResult>, GroupResult>() {
              @Override
              public GroupResult call(ServiceResponse<GroupResult> response) {
                  return response.body();
              }
          });
      }
docs: Divide candidate faces into groups based on face similarity.
  
  @param faceIds Array of candidate faceId created by Face - Detect. The maximum is 1000 faces
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the GroupResult object