To play Ghost Hunters Corp multiplayer, you will need to hit the ESC key on your keyboard to open a tablet menu.
When this tablet gets opened by your character on screen, you will able able to host a multiplayer game by going to the local coop or Steam coop.
If you are playing on Steam, you will be able to invite players to your game by using the Steam coop mode.
You will also need to create a Steam Lobby, but after this, you can invite either by sending them a friend code or through the friend's tab on Steam.
For local coop, you will need to host your game, and your friends or whoever you want to join will need to use your IP address to join your server.
You can figure out your IP address by typing what's my IP into your internet browser.
That being said, which multiplayer mode should you use? First, local coop is more or less for players on a local network or connected through a program such as Hamachi.
Because of that, It's highly recommended to use the Steam client as your means of connecting to friends or hosting games as it is more accessible to players of different regions.