Easy Achievement to unlock.
You can find survivors after completing side missions.
After completing a side mission and having someone join your party, rather than take them all the way back to a bulletin board, simply drive away from them, after around 100 meters an on-screen message will pop up saying that they have been added to your Bulletin Board.
Do this 5 times and the achievement is yours.
**UPDATE** - it does not count if your the second person in the game. only the primary person. found this out the hard way. Credit Goes to oxy71 for finding this.