The latest update on the Windows version of Borderlands 2 has disabled cross-play for Mac and Linux. Currently, Mac/Linux players are unable to connect with Windows players.
We are working on an update to bring the Mac and Linux versions of Borderlands 2 back in sync with Windows, but we do not currently have an ETA for release. We encourage players to join our #Borderlands Discord Channel to coordinate co-op matches with other Mac players, as we work towards delivering the update to re-enable cross-platform play with Windows and Mac players.