If there are multiple demon hunters in my game running Cull the Weak, will the effect stack? Does it stack additively?
The other demon hunters definitely won't get the bonus as it is a passive skill which only affects you. The tooltip states
Increase damage against Slowed or Chilled enemies by 20%.
Which implicitly states your damage. If it read "Enemies you Slow or Chill take 20% more damage" then it would apply to other players in your group.
However, if someone else slows or chills an enemy, you still do the 20% increased damage to that enemy since it doesn't state you have to slow or chill them, only that you are the sole benefactor of the passive skill.
If you were to use another source which added damage against slowed or chilled enemies, such as
Then it would stack additively. So you would do 40% more damage versus enemies slowed or chilled, and 20% more damage versus enemies who had some sort of impairing effect. I also would highly recommend using this gem to pair with Cull The Weak.