I want to play online multiplayer on steam with my girlfriend, but can't find the invite code. Where is it?
The invite code is given to you by the person hosting the game.
Once the host has the game set up, the invite code can be found in the options menu, under the Multiplayer submenu:
If you're friends on Steam with the host, you don't even need to enter a code if the host has the game open for friends, like in the picture above (which is the default).