From the install guide,
>0. Find your Into the Breach directory. If you are playing ITB under Steam, this directory can be found via Steam Library -> Right Click Into the Breach -> Properties -> Local Files -> Browse Local Files.
Failing that, you could try using a file searching program (I personally use Everything) and search for Breach.exe, which is the game executable and is in the same directory as the game files.
That reminds me of co-op Card Hunter. It's good fun; but a lot slower than playing single-player. (That's not necessary a problem; but I does reduce the player-pool down to just those who are patient enough...)