As you cruise around the game you will spot survivors fighting off huge groups of zombies and you will be prompted to help them out by clearing out the zombies. They will usually appear directly in front of you, and a running-man icon with the range to their location and the survivor name will also pop up on the screen to tell you where they are. You will need to find and save at least 15 of these people for the achievement to pop. Most of the survivors are alone, however there are at least two three-person groups that spawn on the freeway in the very middle of the map through the duration of the game. There are far more than 15 survivors that appear, however you might as well free every one you come across if you have the few seconds to clear the area and get yourself some easy PP to level up.