The Wretched were a large group of people that traveled from afar to The Citadel in search of a better life.
After Immortan Joe took over the aquifer and turned it into The Citadel, the news of its vast water and food supplies spread across The Wasteland. Many people, including whole families decided to travel to The Citadel in search of a better life. Plagued by disease and hunger they lived in makeshift tents, feeding off of maggots and trading or holding onto whatever little possessions they had.
Gathered at the feet of The Citadel they rely on Immortan Joe's occasional supply of water, fighting for it more often than not. They also hope to be taken aboard the elevator platform up to The Citadel. Only the people useful to Immortan Joe are taken up. Healthy children, potential breeders and those fit enough for manual labor.
Post Fury Road
After overthrowing Immortan Joe, The Citadel offered water for The Wretched and took up whoever could fit onto the elevator. Bearings of Immortan Joe's cult were destroyed and The Wretched formed a small town at the feet of the former Citadel where trade and prosperity was now present. The citizens of the former Citadel would be taught about historical events by The History Men.