The Horde Bonus is an advantage that some units of the Chinese army would earn when in a group of five or more, increasing their rate of fire by 25%. When the Horde Bonus is active, there is a red star visible beneath the affected units.
It is unknown what the rationale behind the bonus actually is, but may be a result of troops' increased morale and fervor in battle.
Any combination of the above mentioned units would activate the Horde Bonus. Garrisoned infantry benefit from this bonus, as long as it was active when they entered. Thus, for a building garrisoned one by one, it may be beneficial to evacuate the troops, wait a moment for them to gain the Horde Bonus, then immediately re-garrison them.