To expand on it a bit, Light and exp gain are split between all people that kill a mob and sometimes if you don't do enough damage or get enough aggro, you'll get no credit, it's far less efficient for multiple parties to be attacking the same mobs.

You can easily test it. Kill a mob and see how much EXP you gain, then do it again and have someone outside of your party attack it, you'll get less. I can also effect if you get a chain or not.