Anyone who's done open world events like Vinewrath has heard a lot of conflicting claims about how events scale and -- in particular -- the scaling effect of AFKers and folks lying dead at a boss. It's also been a debate on Reddit for the past month or so.  Now a guy on reddit has tested and provided some solid video evidence for how it actually behaves.  
BTW, event scaling can take a few forms: Total boss health, # and level of adds, the skills the boss uses, etc. I saw one video where EACH AND EVERY dead person waypointing away from one of the Vinewrath bosses dropped the boss' max health about 2%! Imagine how much easier it is to win when 20 dead people WP away!
Here's a summary of what we know now. First, UPSCALING:
- Events scale up when you are in range of them. The range seems to be ~1800 -- about when you see the event pop up in your quest tracker.
- Everyone in range scales the event. Dead, AFK, just passing through -- it doesn't matter. If you are in range of the event, you are scaling it up.
- New events upscale immediately. If you happen to be in range when a new event spawns, you are scaling it up. Dead, AFK, just passing through -- again, it doesn't matter.
- Ranger pets don't upscale events. Presumably this applies to necro pets too. It seems just players count.
- The first few folks don't scale the event. It appears to vary by event, but it's a small number (perhaps one party of five for most events). So, if you are doing an event with 5 or fewer people, it doesn't downscale the event when you waypoint.
- Downscaling happens immediately when players move away. Run out of range, or waypoint away.
- Moving away downscales, even if you've tagged the boss. And if you've tagged the boss, you still get credit for the event even after you move away.
- If you are defeated, you'll help the event succeed by waypointing. This is the bottom line. You can save yourself a couple silver by laying there dead, but you risk getting nothing because you are upscaling the event for the folks still alive. You might also experience a negative reaction from those still alive trying to beat the event.
- If you are AFK or just passing through, you'll help the event succeed by moving out of range.
- These results match my own experience in the Silverwastes and elsewhere in open world PVE. Boss events definitely get much easier when dead folks waypoint. (And fail far more frequently when they don't.)
- The data in the videos linked above were collected using a DPS meter mod that's in violation of the TOS, so we can't verify them independently without risking our accounts. :(
- These data seem to contradict what ArenaNet told us about downscaling a couple years ago.