As you progress through the story you'll eventually gain an Assassin by Liberating an area of either Boston or New York - Once you have these Assassins it works in a way similar to the previous installments: Pressing LB to call upon your Assassins to help out in fights.
You only need to get ONE (1) Assassin to level 11 (1,100 exp) for the achievement to unlock.
You can earn EXP in two ways.
1. Holding LB and then Pressing X will bring you into the Assassin's Guild screen - here you can send away your Assassins on missions, depending on the difficulty your Assassin can recieve exp ranging from 40 - 100 per mission. (Maybe higher)
The nice thing about this though; your Assassins can no longer die like in previous titles; just become injured - so don't worry if the success rate isnt 100%.
2. The second way of getting exp is to simply summon the assassin in battle; I'm not sure if they get exp for kills or by simply being summoned but either way, use the ability enough and they'll level up quickly.