Add Particle System to a Group Instance?

WARNING: Some people got freaked out by the awesomeness of my dumb questions and incorrect language. So please ignore the little mistakes, missing knowledge, be sure that I´ve googled my questions before with no results and, please, answer in a gentle way. Thanks. :slight_smile:

As long as I know a Group Instance is an exact copy - not duplication - of another object which uses up less memory because Blender doesn´t have to read he geometry data twice.
But if a Group Instance copies the exact geometry data, why can´t I add a particle system? A particle system (hair) just needs the geometry, not the object itself for producing an hair system (not talking about hair modifier!!).
Is there any way to add a particle system without adding it to the original object?

Thanks all of you beforehand, and I wish you a happy new year!

Treat the group instance as a visual representation in a point in space of the original grouped objects (an empty that looks like the group). It does not exist as a mesh so you cannot edit it or treat it as an individual object

What do you actually want to achieve, give specific examples, demo files etc

I want to create rocks. Many, many rocks in my scene. So I want to model, at least, about 6 rocks. Than I would use Group Instances to fill the scene and save memory.
But I want some (!) rocks, not all of them, to have moss on. I found a pretty realistic looking and memory saving way to create moss using a particle system; but I can´t attach one on a Group Instance. Creating new rocks with moss would increase the memory usage (and my GPU can´t handle that much…), and it would look boring, because the moss appears only on the same rocks.

That´s why I asked it here. :confused:

Mh … this seems to be a known problem.

Trying different workarounds, to create a forest, I try to push towards a solution, which minimalizes Memory footprint and computation on rendering.

Sure, I know about pre-rendering trees (using a rendered tree on a plane to save memory). But, thats not the kind of solution I like to approach atm.