It's pretty easy to get hired as a CS70 grutor, because they need a lot of them. You have to fill out an application. I joined a week late, and didn't have an interview. I emailed the professor I had for the class and asked if I could be a grutor.
Fill out the google form with your top choices. It's definitely hardest to get CS 5 or small electives taught by cool profs, but big classes like CS 70 are generally less competitive.