Is there a level gap that kills xp? I have not noticed one thus far. I do know you can be to high for mobs and not get skill points yourself, but does it hurt the ones you are with? For instance, could I play with my friends at a higher level and not kill the xp for them?
Could I be on the low end of a group and not worry about xp?
I can't tell you exactly if level gap will kill the xp you get from each mob since I'd only party once, and the chat window scroll up so fast that I couldn't even see how much xp I got from each mob. However, as for now, the amount of xp you get seems based upon the amount of damage you dealt, so if you have a class rank 10 in your group while everyone else is class rank 2 or 3, the rank 10 person may kill everything in two hit or so, thus you may not even able to touch the mob before it dies. Nonetheless, I still won't judge the party system by this, I think it's just the matter of balance and I think SE will iron this problem out before release.
On the other hand, when the game is actually released, I think people would tend to group up with players around the same level. As a rank 10 player, I could only get maybe around 100 xp from killing a low level mob, so I really don't see the reason for a rank 10 player staying at low level area and look for group.