EDIT

The ERA 300’s cannot be set as surrounds on the Ray but can be grouped with the Ray.

https://support.sonos.com/en-gb/article/surround-sound-guidelines-and-limitations

When grouped and playing content from the TV there will be a small delay on the 300’s which could be irritating if in the same room.

what about content from Spotify only. Can era300 and ray be coupled together?

Yes, any speaker grouped to another can play the same content.  So if you were to connect by Bluetooth to the ERA that content can play on the Ray when the two speakers are grouped together.  Same applies to any speaker in your system.

Same applies to ALL music services such as Spotify, BBC Sounds, Apple Music etc

Just to clarify here, era 300 speakers cannot be bonded as a HT Surround with the Sonos Ray - it can only be used in a HT setup with an Arc, or Beam (gen2), but it can be ‘grouped’ with a Sonos Ray. See this link on how to group/ungroup Sonos rooms…

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

One Era can be grouped with the Ray, two could not be bonded as surrounds, because the Ray does not do Atmos. Grouping will give you some delay on the Era while playing TV- sourced sound. Music will be fine.