code: public Observable<ServiceResponse<GroupResult>> groupWithServiceResponseAsync(List<UUID> faceIds) {
          if (this.client.azureRegion() == null) {
              throw new IllegalArgumentException("Parameter this.client.azureRegion() is required and cannot be null.");
          }
          if (faceIds == null) {
              throw new IllegalArgumentException("Parameter faceIds is required and cannot be null.");
          }
          Validator.validate(faceIds);
          GroupRequest bodyParameter = new GroupRequest();
          bodyParameter.withFaceIds(faceIds);
          String parameterizedHost = Joiner.on(", ").join("{AzureRegion}", this.client.azureRegion());
          return service.group(this.client.acceptLanguage(), bodyParameter, parameterizedHost, this.client.userAgent())
              .flatMap(new Func1<Response<ResponseBody>, Observable<ServiceResponse<GroupResult>>>() {
                  @Override
                  public Observable<ServiceResponse<GroupResult>> call(Response<ResponseBody> response) {
                      try {
                          ServiceResponse<GroupResult> clientResponse = groupDelegate(response);
                          return Observable.just(clientResponse);
                      } catch (Throwable t) {
                          return Observable.error(t);
                      }
                  }
              });
      }
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