Hmmm... I had a feeling you might say that, but didn't mention it because I couldn't think of a workaround.
Best I can suggest with an SO is to collapse all groups other than the source group, and Alt (Opt) drag the source layer to the target group. That will produce a copy, as evidenced by the double cursor. I'm not sure if that has any advantage over copy and drag, but it would, at least, let you keep your eye on the target layer.
I'm afraid my illustration has uploaded a wee bit bigger than life size, unless you are using Windows 95 on an antique VGA screen