code: private static void addCampaignsToGroup(
        AdWordsServicesInterface adWordsServices,
        AdWordsSession session,
        CampaignGroup campaignGroup,
        List<Long> campaignIds)
        throws ApiException, RemoteException {
      // Get the CampaignService.
      CampaignServiceInterface campaignService =
          adWordsServices.get(session, CampaignServiceInterface.class);
  
      List<CampaignOperation> operations = new ArrayList<>();
      for (Long campaignId : campaignIds) {
        Campaign campaign = new Campaign();
        campaign.setId(campaignId);
        campaign.setCampaignGroupId(campaignGroup.getId());
  
        CampaignOperation operation = new CampaignOperation();
        operation.setOperand(campaign);
        operation.setOperator(Operator.SET);
        operations.add(operation);
      }
  
      CampaignReturnValue returnValue =
          campaignService.mutate(operations.toArray(new CampaignOperation[operations.size()]));
      System.out.printf(
          "The following campaign IDs were added to the campaign group with ID %d:%n",
          campaignGroup.getId());
      for (Campaign campaign : returnValue.getValue()) {
        System.out.printf("\t%d%n", campaign.getId());
      }
    }
docs: Adds multiple campaigns to a campaign group.