There are three teenagers that need to be saved for this achievement, and they're all in the prologue. Each of them will have an SOS comic-style bubble over their head, and they will each be pointed out to you as you progress. It is extremely hard to, and very unlikely, that you'll miss them.
The first two are easy, simply kill the zombies that come out until the cooler can rescue them (it only comes after you finish killing the zombies in the area). For the last one, it's broken up into three sections.
First section, kill the zombies until he gets off the pole, then kill some more. Break through the gate, and you'll get another hoard of zombies to kill. I recommend saving your Star Soul Mode (right trigger when the sparkling meter on the bottom left of the screen is full) until after you break through the gate, it makes this second part 100x easier and you can get a bunch of Sparkle Hunting combos at this point. Just keep an eye on his health, if you don't do this part right he'll most likely turn into the zombie. After you finish this wave, avoid the truck, then finish off the last set of zombies.
If you fail to save one of the survivors and realize it right away, fret not! In that case, you're welcome to restart from the last checkpoint to try saving them again, without losing the achievement. Just know that restarting takes points off your final score.
Happy survivor hunting!