According to Beat Games: https://beatsaber.com/faq.html
Beat Saber multiplayer supports cross-headset play within the Oculus ecosystem, so you can play with friends regardless of whether they own a Rift, Quest, or Quest 2. Currently, cross-play across Steam, Oculus and PlayStation platforms is not planned.
So Steam and Oculus versions can't do multiplayer together.
As someone who actually own Bs on steam and quest i can confirm you can only play with other Quest users with the Oculus store Quest version...............BUT, all is not lost, you can make the game crossplay with the steam version via sidequest and then installing the Beattogether mod, and that will allow your Quest version to play with steamvr versions.......and yes it is not the besy solution but it is all Quest users have at this time, so hope that clear things up for you. 😉
Oh and if you have a Quest you should have sidequest on it by default as it is too good a mod not to have on your Quest. 😉
Hmm, what happened to my post? The signature somehow got merged into the middle of the text.