I'm trying to wrap my head around this...are light groups just different render passes that get "embedded" in the final output--and the compositor is just manipulating these? Or is Blender using the scene data somehow?
I'd say yes, it's just like render passes for you to have much better control over your scene at the compositing stage. And I guess yes as well to Blender using your scene to derive all the information. I'm not sure if that answers your question? It's more like an affirmation.
It's more of a render pass where you tell blender I want the combined render only using these lights. Then you can use that data to manipulate in the composite.