I've been trying to get online with a GFWL game with my friend for the past few days. We both have legal versions of GTA 4, LCPDFR installed, and the exact same mods and vehicles as well as the Rockstar 1.0.7.0 update. I can start a multiplayer game just fine, multiple people are able to enter when I leave entry open (I don't know how they find it but they are able to get in). I send my friend an invite with GFWL and he gets an error Unable to Join This Game, I get the same error if I attempt to join a game he creates. However I have been able to invite others to my games in the past, as well I can join games created by other users just fine. It appears this issue is just between us and I have no idea what is causing it anymore.

Are you both located near each other, or on different sides of the world?

We are located in the same state. An hour and a half driving distance.

Sorry for the bump but we are still trying to figure this out.

What is your NAT type on GTA IV?

My friend's is strict. He made a few changes to his firewall however it still isn't working yet. We do believe that the NAT is the issue. I will update this post if this solves the issue for us.