It gives you an error message when you try and join another person's party, unless one of you is plugged directly into the modem. Trying to go through a router messes it up, i've tried opening the ports and everything but it still doesn't work. A friend of mine has it for both the 360 and PS3, when he plays on the 360 his NAT Type is set to open and he can connect with friends, but when he plays on the PS3 its is set to Strict and cannot. So obviously, it isn't just a problem with our individual routers. I had the same problem with WAW, and they never fixed that either.