Everyone sends in their resumes... the good ones are contacted by producers and interviewed. Once completed they pick the top ones and interview them again... final casting call and the producers make the final decision.
Im sure most of them are picked to enhance the drama on the show, with a few people they think would be good choices.
It's possible a friend of a friend of a producer would recruit a few of the contestants. Most send in video tapes then are brought in for a few casting calls and psuedo interviews.