This is an old problem, but I cannot remember seeing any posts about it here (too lazy to search further than the first page as well).
Sometimes, in missions, teammembers become invicible. I can not see them, but they see me. Or the other way around. Kinda annoying when you ask them where they are, and they tell you "right in front of you, man" and they think I am a fool because of it.
One trick that fixes the problem (until next time it occurs), is for the invicible player to move out of visual range of the rest, and go back again. That usually helps.
Oh, while we're talking about invicibility: Sometimes, mobs are invicible in the same way too. Thank god for autoattack (which works 50% of the time when dealing with invis. mobs)