It doesn't affect your xp or drop, but it can affect that players xp and drop. It depends on whether or not the other player did enough damage to the mob or not. If you hit a mob first, and that mob dies because you and other players are wailing on it, you get full xp regardless of how much damage you did during its life.
So someone can power level me? For example if I'm lvl 5 and start a fight with a lvl 30 mob and then kill by my friend who is lvl 50. In this case I still receive full xp?
While it's not really what SE wanted to see happen, and it is slightly frowned upon, you will NEVER get punished for someone killing your monster in any way even during power leveling. If you hit it first, you get full credit and drops.
If anyone else helped you, regardless of their level, you still get full credit and drops, and if the person did enough to contribute themselves, he/she would get exp and his own set of drops as well.
It's the most fair thing really. I kinda like being able to do hunts just hitting whatever and not fighting over claimed mobs.