Group: The fully-qualified URL of a zonal Instance Group resource. This instance group defines the list of instances that serve traffic. Member virtual machine instances from each instance group must live in the same zone as the instance group itself. No two backends in a backend service are allowed to use same Instance Group resource.
Note that you must specify an Instance Group resource using the fully-qualified URL, rather than a partial URL.
When the BackendService has load balancing scheme INTERNAL, the instance group must be in a zone within the same region as the BackendService.