Yes, sorry, we tend to use “box” for the thing nymea runs on, in your case the RPi. I should train myself to use the correct term, which is nymea:core.
I totally agree that it’s about time to implement grouping. For now you could do this to work around it:
Go to Magic and create a rule which executes on/off on all the lamps to you want. Then, at the top, there is a gear right next to where you enter the “magic” name. Expand that and check “This rule is executable”. Then you’ll see a button appearing in the Scenes screen which you can press to activate. You could create a bunch of “Room x on” and “Room x off” scenes with that.