Update May 19:
Mac Steam and Windows Steam games should now be in sync! If you're still experiencing issues joining cross play multiplayer games, please make sure that your game is up to date.
If you are still experiencing multiplayer connectivity issues, please submit a bug report to us here.
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Civilization VI crossplay is temporarily unavailable between Mac and Windows players. Mac players will not be able to connect to Windows players, and vice versa. Our team is working to bring the versions back in sync as soon as possible.
We'll update this article when we have any new information. Thank you for your patience.