There are 15 missions that you need to complete to recruit the Legends of '67 and La Moral, and thus get the achievement. They are found in the El Este region. All are story based and unmissable.
1. Meet the Legends
2. Take Your Medicine
3. Pics Or It Didn’t Happen
4. The New Revolution
5. Bullet Points
6. Knuckleball
7. National Treasure
8. Room Service
9. Harpoon
10. Our Right to Party
11. The Lion’s Roar
12. Tourist Trap
13. Surgical Strike
14. Weapon of Choice
15. The Deported
Edit: There have been reports that if you play in co-op only the host will unlock this achievement.