Associated Content has an article that outlines how you can get cast as an extra in a film production. A few excerpts:

“Here’s how it works. First you have to find and register with a reputable extra casting agency. You may have seen them online or advertising in your local papers. For the most part you want to stay away from those.”

“Any site or so-called agency is trying to sell you head shots or portfolios to become an extra, run. You are being scammed. You will only need one or two photos at the most and a Polaroid or digital photo will do.”

“…most just need bodies to walk down a street or sit in a crowd. Extras don’t go to wardrobe or makeup for the most part. Extra work pays in the range of $75.00 to $125.00 per day. Like I said you will not get rich, but it’s a fun way to earn extra cash.”