I agree with GH.
For me, I absolutely never let horses eat while I'm anywhere near their face/holding the lead rope. If I'm tacking them up or grooming, sure they can eat. But any kind of grazing while attached to me or eating hay from my arms while I place it out is entirely off limits. I will very rarely allow hand-grazing but I will grab the horse's halter and lower their head to the grass, to say "ok, you can eat" if I decide to let hand grazing take place - they never get to just start eating, even if I'm about to let them hand-graze.
I just figure that a dominant horse never allows to 'lesser' horses to eat without his/her ok ["ok" in the form of not chasing the less-dominant horses off]. Those less dominant horses don't get to just come up to a more dominant horse's pile of hay and just start munching either. They stay off to the side where it's easy to get away and very cautiously approach the more dominant horse, if it's been previously proven "safe" to share hay with the dominant horse. If they know that more dominant horse WILL chase them away, they generally won't even try to sneak bites from their superior.
Friends of the dominant horse might get to be a little less cautious but that's because 1. They're friends 2. The subordinate friend is well aware of their position as the subordinate. Same for me - horses that respect me and willingly do what I ask around food get rules that are a bit more 'relaxed' than horses that are constantly being pushy around their food. Everybody starts out with the same "you get AWAY and stay away from that hay when I ask! I don't care what you're doing, or how hungry you are, I'm telling you that's my hay" rule though!
Fabio - 13 year old Arab/QH gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.