code: @BetaApi
    public final Operation addInstancesInstanceGroup(
        String instanceGroup,
        InstanceGroupsAddInstancesRequest instanceGroupsAddInstancesRequestResource) {
  
      AddInstancesInstanceGroupHttpRequest request =
          AddInstancesInstanceGroupHttpRequest.newBuilder()
              .setInstanceGroup(instanceGroup)
              .setInstanceGroupsAddInstancesRequestResource(instanceGroupsAddInstancesRequestResource)
              .build();
      return addInstancesInstanceGroup(request);
    }
docs: Adds a list of instances to the specified instance group. All of the instances in the instance
  group must be in the same network/subnetwork. Read Adding instances for more information.
  
  <p>Sample code:
  
  <pre><code>
  try (InstanceGroupClient instanceGroupClient = InstanceGroupClient.create()) {
  ProjectZoneInstanceGroupName instanceGroup = ProjectZoneInstanceGroupName.of("[PROJECT]", "[ZONE]", "[INSTANCE_GROUP]");
  InstanceGroupsAddInstancesRequest instanceGroupsAddInstancesRequestResource = InstanceGroupsAddInstancesRequest.newBuilder().build();
  Operation response = instanceGroupClient.addInstancesInstanceGroup(instanceGroup.toString(), instanceGroupsAddInstancesRequestResource);
  }
  </code></pre>
  
  @param instanceGroup The name of the instance group where you are adding instances.
  @param instanceGroupsAddInstancesRequestResource
  @throws com.google.api.gax.rpc.ApiException if the remote call fails