Hy guys, I'm Italian, so please apologize my little english XD
Last night i tried to play a co-op game with a friend on halo: sp. We both have NAT open.
I invited him in my lobby, he accepted the request so he entered the xbox live group and... nothing... He DIDN'T enter in the game lobby.
We had tried to exchanging parts, but nothing the problem still remains...
We have this issue in Halo:sp and in Killer Instinct....
here's a video that i've made yesterday (is in italian, but you can equally understand)