What are the requirement to gain experience in Guild Wars 2 when killing a mob?

What makes me gain experience when I kill a mob? Sometimes I try to help other players and when I kill a mob and I gain nothing. Why? Should I engage the fight? Is there a minimum damage? What is the rule?

Solution 1:

I found my answer here

"Furthermore, unlike even games with mob tagging and forced grouping, both players will receive loot from the mob all based on a separate loot table as well.

There is no mob stealing, there is no loot stealing, both players receive a reward, both players receive experience, and both get credit towards quest fulfillment.

It should be noted of course that this mechanic will have a limit in place. This limit will be that if a mob is below a certain threshold of health and near death, than the player that helps will not be rewarded."