Legendary NPCs are usually the Captains/Polemarchs at enemy Forts. You can use your eagle to see if an enemy is legendary. Press Up to use eagle view and L2 to scan the enemies. In the top right corner, it will display the NPC rarity. The ones you want are colored yellow. Simply go to a fort and scan everyone before attacking. To recruit the NPCs, you must knock them out. Either use the Sparta Kick skill as the finishing blow or unequip your weapon in the inventory to fight with bare fists. You can use weapons to weaken the enemy, just the final hit must be from bare hands or Sparta Kick. When the enemy is knocked out, stand over them and press L3 to recruit. You cannot do this while in combat. If other enemies are still alive, kill them, then recruit the knocked-out enemies. Then, open the ship menu, select "Special Lieutenants", and hire the NPC for some money.