Set opacity for the whole group/artboard

Hi there, how can I set/animate opacity of the whole group or artboard? At the moment lowering opacity affects each child layer.

Let me give you concrete example: I use artboard as a container for my illustration, let's call it illustrationArtboard. I duplicated illustrationArboard multiple times on the main animation artboard and I'd like each instance to have different opacity. However, tweaking the opacity changes opacity of each layer so if the illustration was a pig, then I can see through pig's nose and face and it al looks weird. Hope that makes sense.

Thanks