code: func (gce *Cloud) AddInstancesToInstanceGroup(name string, zone string, instanceNames []string) error {
  	if len(instanceNames) == 0 {
  		return nil
  	}
  	// Adding the same instance twice will result in a 4xx error
  	instances := []*compute.InstanceReference{}
  	for _, ins := range instanceNames {
  		instances = append(instances, &compute.InstanceReference{Instance: makeHostURL(gce.projectID, zone, ins)})
  	}
  	op, err := gce.service.InstanceGroups.AddInstances(
  		gce.projectID, zone, name,
  		&compute.InstanceGroupsAddInstancesRequest{
  			Instances: instances,
  		}).Do()
  
  	if err != nil {
  		return err
  	}
  	return gce.waitForZoneOp(op, zone)
  }
docs: // AddInstancesToInstanceGroup adds the given instances to the given instance group.