Throughout the game you will come across people who are surrounded by hordes of zombies. You'll get a prompt on the screen for these side missions. It will say Kill the zombies surrounding "name here" Just kill all the zombies and you'll get a nice PP bonus for saving them.
This is nothing you need to search for this happens pretty often.
EDIT: if for some reason at 15 this doesn't unlock for you. some information from Br0modoDragon
You may want to add this to your guide, the progress bar for the actual achievement was at 100% and remained locked, even after having rescued 15 survivors (counted them as I went). The achievement is now unlocked, to my understanding if you are to rescue 15 prior to having the phrase "rescue the family" come up on your screen upon entering the south-east area of the map, it will not unlock. After having this sub-mission appear I continued in hope of a delayed achievement. Upon completion (all family members rescued or dead) the achievement popped. I'm certain it was not a miscount both due to the 100% mark on the achievement itself and because the final family member rescue resulted in their death preceding the achievement. Feel free to throw this into your guide, I'd love some props on it ;) if anyone else experiences this please comment so we can confirm this as common an not just a glitch in my personal game!