After GameSpy closed it's doors this game has lost its ONLINE play.
Using Hamachi and other similar VPN/Virtual LAN solutions is both complex to set up and intrusive to the end user.
To the rescue comes OpenSpy - an emulated gamespy server.
It's hosted by a third party so its availability might not last forever however at the time of writing it is working brilliantly.

The easiest way to do this is hex-edit the file responsible for connecting to GameSpy and make it connect to OpenSpy instead.

1. Right click your game in your Steam library list
   
2. Select Properties
   
3. Browse Local Files
   
4. The file you want is
   
   Hunted\BINARIES\WIN32\P4DFTRE.dll
5. Either edit that file following the steps outlined here[beta.openspy.net] and doing steps 1,2 and 4.
   As an alternative you can download the already fixed DLL from here[mega.nz] and replace it.

1. Start the game and finish the tutorial (by the point you get to the first town and are asked if you want to continue or go to the menu) if you haven't done so yet.
   
2. Select Adventure
   
3. Select Online
   
4. Select Register
   
5. Type in your desired Username and Password (remember those as you'll need them later)
   
6. Select Login and use your username and password used during registration.
   
7. Go to friends and add your friend there

IMPORTANT: Don't use the same combination of username and password you use in other logins.

Your coop partner should do the same. Only one needs to do the last step and the other needs to accept the friend request while logged in and inside the friends list.

1. Launch your game
   
2. Select Adventure
   
3. Select Online
   
4. Login to "GameSpy"
   
5. Select Private Match
   
6. Press F1 to Invite Friend
   
7. TAB to Manage Friends
   
8. CTRL to Add Friend
   
9. Type their name (GameSpy username) and Enter to Accept
   
10. Once your friend has authorised your friend request click them in the Friends List to invite them to your game. IMPORTANT: Never click twice on your friend's name or the process will bug and then both of you need to re-log into gamespy.

NOTE: Steps 7 to 9 are done only once.