State of Decay

Shortly after reaching the Church in the story, you'll be asked to head to the radio tower near the center of tower to scout for supplies. Once you climb the tower you'll hear of a nearby group of survivors that you can now go help. Scan the entire town prior to leaving the tower, especially the hordes of zombies to make your trip back to the Church safer. Head across the street from the tower to meet the group of survivors in the Enclave and ask their leader to join you. Escort them back to the Church for the achievement to pop.

If this doesn't work for you for whatever reason, there is another enclave near the church that will constantly be under siege, and you can go defend them for easy points. Helping out lost scavengers works as well, and you can see your progress toward getting them to join you by looking at their base on the map and seeing how full the "friend" bar is.