There are 13 missions that you need to complete to recruit the Monteros, and thus get the achievement. They are found in the Madrugada region. All are story based and unmissable.
1. Meet the Monteros
2. Clear the Air
3. Second Son
4. Packing Heat
5. Napoleon El Pequeño
6. Blood Ties
7. Sundown
8. Bury the Hatchet
9. Open Skies
10. Diesel Daisy
11. Fly Ball
12. Death Warrant
13. Justicia Montero
Edit: There have been reports that if you play in co-op only the host will unlock this achievement.