Since Rhino 7 you can set transparency for objects or layer color, so technically it is possible to make your Group 1 layers have some transparency. I don’t think it is possible to edit many of them at once for transparency only, unless you use a script. Please try this:
SetLayerColorTransparency.rvb (816 Bytes)
(save on your HD, drag-and-drop into Rhino, from now on an alias will be added:
It will set transparency for any layers currently selected in the Layers panel.
I am not sure how practical this is to get an overlay transparency effect you are asking for, but this is as close as you can get in Rhino currently.
EDIT: this will only show properly if all your objects on these layers have color set to ByLayer.
If they are by object, you either have to change them to ByLayer or the script needs to get smarter to pick up per-object colors (doable, but a bit more work)