The people in your fort are called Survivors. They each need a house to live in, and eat 1 unit of food per day. Every survivor has five skills, a happiness level from 0-100, can equip one weapon and one nonweapon, and can have up to 4 perks. When a survivor is not on a specific mission, they will return to the fort and do guard duty.
New survivors can be gotten through recruiting missions, and sometimes they'll appear on their own via random events.