To recruit a follower as the Saracen, you must land on a space where another character is currently standing. That character must have at least one follower with them already. Then you choose to Encounter the Character(not the Space), and in addition to the regular Battle option you now also have a second option for Recruiting. Click this and you get to roll a die. If you roll a number that is equal to the other character's number of followers or less, you can choose one of their followers to be recruited by you. Once you've chosen a follower to recruit and finish your turn, you'll unlock the achievement.
This means there's a little bit of RNG going on here, and you'll have to wait until the game's got going before the achievement can be unlocked. Also you've got to avoid trying this with opponents that can't have followers with them, like the Ninja. But otherwise it's fairly easy to do.