This game runs at 2 different frame rates, 30 or 60. if they are playing on a different fps you will not be able to join them in lobby matches. There connection will just come up red and the only option is to ignore them. But you can still join them in privet matches just runs like crap.
Im starting to think you can only see people if they are running the same aa as you. I have mine set to 4 and I can see people. But if I turn it off I cant. More testing need to be done. Video settings may have something to do with the netcode in general. As having different setting may cause problems.