When a power has an effect that occurs upon hitting, missing, or otherwise affecting a target, the effect takes place only if the target in question is a meaningful threat. For instance, characters can gain no benefit from carrying a sack of rats in the hope of healing their allies by hitting the rats.
When a power’s effect involves a character’s allies, use common sense when determining how many allies can be affected. Dungeons & Dragons is a game about adventuring parties fighting groups of monsters, not the clash of armies. When read strictly, a cleric’s power might be able to give a hundred “allies” a free basic attack, but that doesn’t mean that cleric characters should assemble armies to march before them into a dungeon. In general, a power’s effect should be limited to a small group of about eight people—the size of an adventuring group plus a couple of friendly nonplayer characters—not hired soldiers or lantern-bearers.
Published in Rules Compendium, page(s) 108.