This achievement is story related.
In the menu, under Quests > Eltariel Quests, you'll find missions where you can recruit orcs. Since Eltariel doesn't brand orcs, you have to help out these particular ones with a battle or mini-quest they're on to get them to join you. One of the first of these is "The Bow of Morgoth" in Cirith Ungol. Since this is the quest I unlocked the achievement with, I'll explain how to complete it:
Note: it should go without saying, but if an orc you're trying to recruit dies you won't get the achievement. I don't think any of the orcs I recruited came close even once to dying so this shouldn't be a problem. But do make sure you kill local captains in each region (under the Fallen Blades quests) first. This will make things easier, especially in Cirith Ungol for "The Thrall" recruitment quest.