No one can survive alone with the ever-growing mass of evolving zombie beasties. With new mutants popping up every night, you're going to need a reliable squad packed with many skills to tackle the horde.
Survivors vary in rareness, skills, and personalities. The main way you can get new Survivors to join your City is by carrying out missions in the daytime which offer the reward of a new Survivor. There are over 50 lone wolves out there that would be invaluable to your chances of surviving the night! You can also buy Survivors in the black market.