The Beam comes with it - you just select the Beam in your Airplay list.

So the two are one airplay speaker (so sound volume controls are bonded also).  Is there a way to airplay the sub solo?  I want to use it for music without the beam. My research suggests it only works when bonded with other Sonos speakers. 
 

thanks. 

The Beam & Sub-Mini are bonded together and the crossover etc; set and the room is then usually Trueplay tuned and sometimes also manually tuned to a users own listening preference in the environment. They two components appear as one ‘room’ in the Sonos App.

You would have to undo all the setup/bonding to use the Sub with another speaker, or room.. so it’s easier to leave all as it. Just select the Beam room to add to your chosen group - there’s no way to disable the playing audio output from the Beam.

The simple answer is to buy another Sub-Mini instead - I have one in our kitchen/diner and one in the main bedroom. I highly recommend the Sub-Mini and the Sub (gen3) too. It’s worth noting that the Sub (gen3) is currently on offer at its lowest ever price aswell.

https://www.sonos.com/en-gb/shop/sub

No, you can not send a signal to only a Sub. It is always bonded to another speaker, and part of that ‘room’.