Add device to a Collector Group

Pretty much. By setting the preferred collector id, you're telling LM which collector the device is assigned to. The collector being a member of an ABCG will mean that overload will be rebalanced.

Also, don't confuse ABCGs with load balancing. It's not load balancing. It's overload rebalancing. In an ABCG, one collector could be 99% loaded and the other could be 3% loaded and this is normal/expected behavior. It's only when the load goes above 100% that rebalancing happens, and only enough to get it down below 100%. Definition of load is a specific number of instances. Not actually based on real performance stats.