All multi hit moves are calculated as if you're in the middle of a triple battle, but with two enemies at your side rather than allies. You can only capture Pokémon after you have defeated four of the five. In addition to this, there is the slight chance that one or more Pokémon in the Horde can have their Hidden Ability
So essentially the moves that hit everyone in a triple battle will hit everyone in a horde battle as it functions the same. I'll have to test it but I think that would mean that with multi-pokemon attacks you will always be hitting everyone.
Source: Serebii Hordes