To unlock this achievement, you must have a total of 5 survivors in your party, including yourself.
This can be unlocked relatively easily while playing the game, as you will accumulate more survivors as you progress. Survivors can be found when searching buildings during the day.
While survivors cannot be lost while searching like in the first game, they can be killed by weapon-using zombies during the night. While fending off the wave, prioritize killing zombies that hold any weapons (such as axes and machetes).
The achievement should pop once you have found 4 additional survivors!