Do you mean Grouping or Pairing?  Pairing is normally used in the context of Rear Surrounds in a home theatre setup or when creating a Stereo Pair.  Grouping is when you simultaneously want to output the same music in multiple rooms

Thank you.  I believe i mean grouping.  I have a Five now, and want to add something on the other side of the room.  stereo would be a desired option, but not required?  

If you want stereo it can only be a matching speaker. 

If you are happy to group just to fill the room a bit better, then any speaker will work. You will have to set them up as different rooms in the App, even if they are physically in the same room. 

In that case you can Group your Sonos Five with any other Sonos device, using the App.  If you want a Stereo pair another Sonos Five would be required. https://support.sonos.com/en-gb/article/create-a-stereo-pair

Thank you.  It seems I should buy a second Five for best options.  Or, do you have an opinion of a better choice for the second speaker?  What do you think of grouping the Five with two ERA 100?

A stereo pair of two Sonos Five’s will provide the best audio quality, I find mixing a Five with a different Sonos device in the same physical room creates a wide mismatch of the audio output.  So in brief, if it’s in the same physical room, then another Five will provide the best solution.  Also they are discounted on Amazon with a 28% discount on Prime Day:

https://www.amazon.co.uk/Sonos-FIVE1UK1BLK-FIVE-Black-Speaker/dp/B0876TVDYG/ref=sr_1_3?dib=eyJ2IjoiMSJ9.LL8luIjEOqs7-teHRTiZiisgYgkN_EvnlPUhubCnnW16fGyGRXvajsAzwcMjHONVnoCmNLWmjIKQOpU-ZHwSmU6yGGo6GFjA0xI0LH8242YS_v4JXIlZ9jnKSqRFkX5D_FGkfBKbkteMsoeK91J3-BD4FYqa9tdWfE-3xhGozsEdbshigyBJPY9MCQpUzBlSdNwkw18KeKNAA6zVHfXQEMjArzrRAjYIA6OFt1-ERTI.4J64lO-qVbv6k3w-nYNL2zzvtL4EWGYPKGyiZlNI0GI&dib_tag=se&keywords=sonos+five&nsdOptOutParam=true&qid=1728395252&sr=8-3&ufe=INHOUSE_INSTALLMENTS%3AUK_IHI_5M_VF

Overall I prefer a pair of FIVE’s in the vertical orientation, however, some room layouts may sound better in horizontal orientation. It’s a personal evaluation.

great.  thank you!

When positioned horizontally a Five plays in stereo, but if you orient it vertically I believe it switches to mono only. 

To clarify, that is applicable for a single Five only, not a stereo pair.