I am sharing my dad's steam account on my computer. He bought/downloaded/installed a game on his user, but I have to reinstall it on my user. How can I get it on my user without re-downloading it, like internally transferring it? Extra info: My computer is a Mac running Mavericks
Regarding the comment on your question, you can change your install directory, but that's more for pushing things onto a different hard drives if your computer has them.
The files you'll need will probably be sitting around in your steamapps directory. Since you're running a Mac, this should be
~/Library/Application Support/Steam/steamapps. For completeness - on a Windows computer, this is usually
C:/Program Files/Steam/steamapps or
C:/Program Files (x86)/Steam/steamapps, and on my Ubuntu laptop, this is
~/.steam/steam/SteamApps/, though your exact Linux setup may vary. The steamapps directory should contain a few subdirectories. Find the ones named after yours your dad's Steam accounts and open them both up.
Stop here, open up Steam, and set the download off for the game you want to install. Leave it for a couple of seconds, then exit Steam. This will drop a directory in your steamapps folder named after the game you're trying to install. Find the corresponding folder in your dad's steamapps directory, then just copy-paste everything across. Restart Steam, and resume the download. Steam should check to see what files are already around and what files are needed, note that all the files are already there, and then let you play your game.
If your dad has a bunch of saved games or some clever keybinds set up, those might be transferred across to you, too. Check everything is in order before you start playing.
This is far from the most efficient way of accomplishing this goal, since you'll effectively end up with two separate installs of the same game. If you're familiar with symbolic links and don't mind turning your filesystem into some kind of Cretan labyrinth then you could probably achieve the same effect with those and without making a double-install. This, however, is a little more complex.
Again, for completeness, I'll outline the steps. Rather than copying the install directory across, drop a symbolic link to the install directory in your own steamapps directory. The tricky part, however, comes when the game's install directory also stores saved games and similar items, since you probably don't want your dad to accidentally beat a bunch of the game for you. To solve this, you'd need to have a complete path to your own saved games directory. Every other file and directory in the game's install directory can be replaced with a symlink back to your dad's directory.