Curly joins in George and Lennie's dream of the perfect little farm for two general reasons: what he offers and what he lacks.
He offers money that he can contribute to the purchase.
He lacks a whole and healthy body that would allow him to fully earn his place (and his peers' respect) on the ranch, and so lacks a place to call his own or fit in. Because of these things, he wants to come with them.
Curley's wife dream was to be an actress/model.
She wanted to be in nice dresses, have people love her, and act in movie.
She was convinced that she is good at her job, after a director said she was a natural and she received a letter to be accepted for her dream.
The reason for Candy joining George and Lennie's dream is because he also wants to get out the ranch.
After they killed his dog, he started to dislike it there more.
He also wanted to be in a nice ranch, like what Lennie and George's future ranch would have been.