The Weak Henchmen Force
Each henchmen in combat with a player character raises that characters armor class by 1. When ever the player hits an opponent, the player does an extra point of damage for each henchmen involved in the melee.
The Strong Henchmen Force
As above, but the bonus is 1d4 damage when the player hits.
Note that this only applies to the player's henchmen in melee with the player. If your henchmen have ranged weapons, even if they are not the henchmen of the PC using ranged weapons, the ranged weapon using PC gains the benefit of the ranged henchmen for the duration of the missile fire. If no PC's are using ranged weapons, then the henchmen will not either due to fear of hitting their bosses.
This provides a reason for the monsters to go after henchmen (they have become ablative armor and damage bonuses).
If your loyalty is high enough (Fanatical) , you may also use henchmen as shields as in 'All Shields Must be Splintered!"