Is it possible to have one device in multiple Groups? My outside light is used for my “Christmas” group lighting but I’d also like it to appear in an “Outdoor” group as well. Is this possible? I tried making a duplicate of the device (zwave light) but it wasn’t in the “Add Device” list for the second group.
Yes. You don’t need to create a duplicate device to do this. In fact, I definitely would not do that because it isn’t necessary.
Look at this screenshot. I’ve already got my “Lamp” device in 2 other groups & it still shows up on my “Add Device” popup for Group3. Try this again. I don’t know why this would behave any differently for you.
I’ll shut up now 
I forgot I changed the Display Name so I wasn’t finding it in the list where I thought it should be.
I probably should update that popup to show the Name field instead since that’s the field widely used everywhere else in the Designer. Right now, that code is shared with the in-app “Add existing device” dialog. It makes sense to show the DisplayName in-app but not so much in the Designer.