Sounds like the symlink didn't work. Not sure why, but the library selector in Downloads+Cloud obviates the need for it anyway.
It could be a permissions issue, the game might not have liked being moved outside your /user folder. Do a get info on your steam folder and give everything read and write permissions. Be sure to click the gear and choose "apply to enclosed items."
If that fails, I would right click on the game in steam, go to properties-->local files, and verify the cache integrity. If that fails, reinstall the game.
Also, are there any other games in your library you can try?
I ended up first reinstalling the game, moved steam to the shared folder, then followed your instructions for giving read and write permissions for everything on OSX user#1 and user#2. Next I added the new shared location in Downloads+Cloud.
I can play the game on OSX user #1 (where the game was originally installed, and moved to the shared folder) but still when I try to press "play" on OSX user#2, it has a popup that says 'preparing to launch borderlands 2...' which disappears after 2 seconds and nothing happens. I was able to verify the cache integrity, the only thing I can tell is different is that the game was originally installed, and the Steam folder was moved to the Shared user while I was logged into OSX user#1
Thanks for your help, appreciate it.
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Also I do not currently have any other games in Steam that I can try.
I am no expert in this, but it almost seems like it is still trying to pull the game data from the original Steam folder from OSX user#2, and not from the Shared folder I created.
In the downloads+cloud > Steam Library Folders, I have listed the original steam folder (0 items installed 0 bytes) but I cannot remove it, says the original cannot be removed. There is also the new shared steam folder (1 item installed 5.95gb) that is the game it just doesn't seem to want to access it.
Thanks for any advice!