As you progress through the game you will be tasked with completing side quests that will sometimes give you the option to add survivors to your survivor board to get them as followers. While looking at the side mission menu on the map screen, look to the right of the mission title to see if it has the survivor/running man icon, which means you can get one or more of the people involved in the mission as a follower. While doing these missions, make sure you pay attention because you can still complete the mission without talking to the person long enough to get them to follow you around. You are most likely to overlook survivors that are tied up or trap, which most likely occurs during the Psycho side missions, but keep your eyes open either way.
Note that this only seems to work for the host in a co-op game.