I tried entering the mac beta, and it won't launch with that either.
If you switch back to the main branch, does yesterday's update make any difference?
If you still have this problem, try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
If applicable, disable Steam cloud support either globally (in the client click on the Steam menu and select Settings, and then Cloud) or just for this game (in the library right click Divinity: Original Sin 2 and select Properties, then switch to the Updates tab and check the Steam Cloud section).
Next, try browsing to the '~/Documents/Larian Studios' folder and rename the 'Divinity Original Sin 2 Definitive Edition' subfolder.
This folder contains the saved games (though in 5 minutes you may not have tried starting a new game before the invite), configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different macOS user account would effectively do the same thing. With cloud support enabled, Steam would just download the cloud copy of your existing profile.
After that, extract the replacement folder from the download below into your '..\Documents\Larian Studios' folder, and see if that will let you start the game again.
https://www.dropbox.com/s/5f7ckbkm1g1iqv7/DOS2DE_Doc_36326602.zip?dl=1The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset and low audio quality, which you can change in the options (manually, or hit autodetect) if this gets the game to start.
If that helps, create a new profile and try joining your friend again, or hosting a game.
If the game still crashes, delete the replacement Documents D:OS 2 DE folder and optionally (since you don't have any saves to worry about) rename the original back again.
