You must complete 3 of the Side Missions for this achievement. Side Mission givers are marked on your map as two crooked bars, and Side Missions themselves are marked on the map and Survival Instinct as green areas. There are nine Side Missions: five in the Soviet Installation area and four in the Geothermal Valley area. Whenever you come close to a Mission giver, a notification will come up saying there's an ally nearby.
The Missions are basically just like Challenges, only you need to report to someone before and after finding everything.