Now; let me clarify here; the following worked for me easy. It may not work for yourselves. But it worked for myself, and a few friends.

First things first.
Port Forward!
RECOMMENDED BOTH YOU AND FRIENDS PORT FORWARD

1) Go to http://portforward.com/english/routers/port_forwarding/routerindex.htm and find your router.
2) Once you found your router, use...Borderlands as an example.
3) Follow the guide in obtaining gateway access etc.
4) Where it gets to the method of adding the Borderlands ports ((7777,27900,28900,28902,28910)) use the following instead;
5) Add Ports; TCP/UDP 27015, 27016, 8766 instead of the example given ((7777,27900,28900,28902,28910)).
6) Continue guide until complete.

Next Things Next.
Game Setup
Let me make something clear right now; THE HIGHEST RANK HAS TO HOST.
((Rank is checked via the Abilities page at the bottom))

1) Go to Campaign; Choose 2 Players; the first level; as a test.
2) Set a password if you like.
3) Once the game has loaded either send your friend an invite over Steam. ((Make sure they are running BRINK))
4) Friend should get the prompt for Join and Password, and then will be invited.

OR get your friend to do the following;

4) Go to Freeplay and then Direct Connect.
5) Get your friend to enter YOUR IP Address, found: http://www.whatsmyip.org/
6) I only know the port information for Steam; someone could clarify this?. Port: 27015
7) Password if applicable
8) Click Search, and Join.

I hope this helps you guys out. I'm not too literate in terms of networking and this is the best that I can provide.

I hope it helps!

Good Luck

-Drazzan-

Very good and exactly the way to do it though some people have issues with getting an invite.
The best way is the freeplay direct connect and if you dont know their ip get them to check either via their LAN connection in their router menu or open CMD.exe and type ipconfig and enter and your IPv4 address is your ip address.

I thought this was going to be a "FIX" not a "here is how to port forward" We.. Well most of us know how to port forward already =/

Well; this resolved all the Steam and BRINK Connection issues for me and many friends.

So in light of that; it is a fix.

this worked for me! thanks for the post! I am using the Netgear dgn3500 router!

they even should do it like you dont need to play solo 2 unlock things thats [censored] even on parkour this 3 star isnt possible in your own

so when you play bemore objective with 3 friends i think you need to earn guns for that