Each ally (including the followers and even summoned pets, like Mystic Ally) affected by at least one of the Monk's Mantras gains +5% increased damage. This does not include the Monk themselves. For every affected ally beyond themselves, the Monk gains additional +5% damage, up to 20% total. This skill has no effect with Mantra of Conviction, as this Mantra does not affect allies.
Predictably, this skill is most useful to a Monk in large multiplayer games: in a solo game or in a group of two players, most of the time, it can reach up to 10% (Mystic Ally + follower or second party member). Inna's Mantra set also benefits greatly from it.
Note that to reach the full 20% effect, at least one affected player must have a summoned minion, otherwise even in a full group the effect will not go above 15%.
Damage buff is not multiplicative to other similar modifiers.