As you complete side missions, the survivors will sometimes decide to join your cause. If you look at the side mission menu on the map screen, check to the right of the title of the mission to see if it has the survivor icon (a running man). If it does, this means that you can obtain one or more people involved as a follower. Keep talking to the person once you arrive at the destination and they will end up accompanying you, however some times they will not join you until after the mission is completed.