@Zombies4EvaDude
If UPnP is still disabled or you can't connect despite it being enabled#
Forward these ports on your router 28960-28961 (UDP) (See How to port forward?)
Whitelist these port in Windows firewall
Make sure nothing will block the connection (your router firewall, your antivirus firewall if you have any etc.)
Once those steps are done, open your game and start a private match. (Parties will not work)
If the match requires 2 players to start you can force it to start anyways by starting a game using the solo button.
Don't worry, it's not actually solo, it will start an online private match that your friends can join.
Alternatively you can use the xpartygo command to force the match to start.
Your friend should now be able to join by double-clicking your name in the Friends List tab of the Plutonium overlay