There are a few things to unpack here.

First, which PLAY:5? There are two distinct generations, which are somewhat different. Knowing which type might change our answers.

Second, are you running S1 or S2? Or perhaps a mix, in which case an S2 device can’t be seen by an S1 device, or vice versa.

How do you have your devices connected to your network? Is one, or both wired with an Ethernet cable, or do they both hang off of your WiFi signal?

Have you tried rebooting your router, followed by a reboot (power cycle) of both speakers?

Are you aware that when playing the TV input (coming in through the optical cable), there will be a 75 ms delay before the sound plays on any ‘grouped’ room?

Have you called Sonos Support to discuss it? If so, what did they say? Sometimes, their ability to see data that we can’t helps shape our suggestions. 

Are you adding the 5 to the Ray? That should work, the other way won't for TV audio. 

Thanks for the help. I think I’ve worked it out. Play 5 gen 2. It works if I make sure the Ray is sitting at the top of the list for sources connected to tv, then apply. Once I’ve done that, I go back in and add the Play 5 under it and hit apply again. This way it adds the Play 5 without any issues.