Okay so this isn't anything new as far as ports, simply a manner of determining your capability to connect.

First ensure all the ports are forwarded in your Router and/or Firewall. Review http://www.portforward.com for assistance if you are unsure how to forward the required ports.

Second, once the ports are setup within your router and/or firewall go to https://www.grc.com/x/ne.dll?bh0bkyd2 to ensure that the appropriate ports are Closed and NOT Stealthed.

To do this, once you've entered Shields Up, you'll see two silver horizontal navigation bars. The bottom bar has "User Specified Custom Port Probe", you want to run this specific test.

On the custom port test in the text box between the navigation bars enter "27015 - 27016, 8766" (without the quotes). Run the test, if the ports list as Closed (you'll fail the test) you should be fine. If the ports list as Stealthed, you need to check your Firewalls whether router or software to turn off stealthing.
Rerun the test if you were Stealthed to ensure the ports list as closed.

This worked for me and my friend, we were able to play as Co-Op where as before with the ports Stealthed and 'open' we were not. I hope this helps people connect via Co-Op.

One last bit of advice...
After you finish playing make sure to re-enable Stealth in your Firewalls.

Good luck and enjoy.

Great instructions, no go for my provided modem/router however =( SMC8014WG.

I run an Internet Security Suite which has a software firewall, so even after setting up the router/modem I was supplied by my ISP the ports listed as in Stealth mode. However, upon disabling Stealth within my Security suite, the ports became listed as Closed.

The lesson to be learned is that Stealth needs to be disabled, whether in a Router(/modem), or a software Firewall such as with an Internet Security Suite or Windows Firewall.

Good luck.