Thanks for reaching out. You can group any other SoundTouch speakers together n the SoundTouch app. This is done through your WiFi network to initiate a connection, so you can have music play throughout your home.
If you're looking to just have a single additional speaker close by, if you have a SoundTouch 20 III you can use the SimpleSync feature with one of our newer Smart Home Speakers. In the Bose Music app you can initiate a pairing of your SoundTouch 20 III and your Smart Home Speaker via a Bluetooth connection.