Is Steam online?
Does Steam list the game as running after it is started?
Try doing a clean boot and then test the game. Log out of Steam, exit the client, restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears (alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock), then restart Steam, log in and try the game.
Try creating a new OSX administrator user account (either with the Documents folder in the default location, or on a different drive, if available), switch to that account and try starting the game from there.
Have you tried different ways to join? Other than Steam friend invites, there is also Direct Connection, with a Server ID (which the host can get in the Connectivity Menu after starting a new game or loading a save) or IP address. In the latter case, it can help if the host sets up port forwarding in their router to connect to the PC (ports 23253-23262 and 23243-23252), and then the clients should be able to connect to the external IP address.
LAN may be an option, but if you are not on the same local network, it would require VPN software, such as Hamachi or Tunngle (IIRC one of them has a Mac version).
If you use Discord, that may also be an option.