Not sure I truly understand your question. S2 devices can’t be ‘seen’ by S1, and vice versa. So, if you’re using AirPlay 2 to an S2 device, you wouldn’t be able to ‘group’ an S1 only device like a PLAY:5 gen 1.

Get an inexpensive AirPlay device and connect it to the s5 line in. 

I’d be curious as to what an inexpensive AirPlay 2 device is. Could you elaborate? I could probably use a few, just not connected to Sonos. 

WiiM mini. 

Huh, ok. Not sure I’d call a hundred dollar device inexpensive, but that term is relative. 

I read that one can add a non-airplay compatible device to a group using the S2 app, but as i said, the option to allow that is greyed out. I’ll try to connect via aux. thanks. 
 

You can group Sonos devices using the Sonos software controller. You can’t ‘group’ using the AirPlay 2 interface, since it doesn’t recognize non-AirPlay 2 devices. And since S2 can’t ‘see’ S1 devices, you can not group the two, unless you add an AirPlay 2 receiver to the S1 device using the line in, and then ‘group’ using the AirPlay 2 interface/controller. You’ll still need to use the S1 controller to turn on the line in, it doesn’t default to on when it receives a signal. 

Since we are talking about AirPlay receivers then I suppose the term inexpensive would apply relative to other AirPlay receivers.