Add the new speaker to your system via System / Add Product

That done, you can Group your new speaker with any other speaker using the app.

Note, however, that there are a maximum of speakers in a Sonos ‘room’, depending on the type. Home theater ‘rooms’ in the software max out at 5.2.2, assuming an Arc is in play. A regular ‘room’ is 2.1 max, but you can always group ‘rooms’ together so they play music in sync. And Sonos doesn’t care how many ‘rooms’ are in the same physical space. In my open plan kitchen/dining/living room, I currently have three Sonos ‘rooms’ in play, which I group together as needed.