Well, mostly you need to be registered with agents who handle background player work. Normally, an agent is a person who, through contacts and lots of time consuming effort networking, sourcing, and direct marketing, finds auditions for actors. If the actor gets a part, the producer contracts with the actor for compensation to the actor. The agent functions as a negotiator on the actor's behalf. Extra work is a bit different.
Agents function as a funnel.
They amass huge files of people who want to do extra work. They are contacted by casting people who pay the agent a fee to send them extra candidates. Any more and you are being scammed. Some agents do not charge a fee to register extras. This is good.