Be on the look out for more Radios as you'll be able to call in more survivors this way.
You can gain Radios in the following ways:
  • Kill walkers around your camp
  • Complete certain Achievements
  • Gain them from Reward Crates after completing a Mission
  • Complete personal challenge goals
  • Complete guild challenge goals
  • Also sometimes from the Trade Goods Shop both for free or Trade Crates