Membership

www.fabcastingagency.com/

Agency - they don't charge for a membership (I presume).

I'm afraid I don't know much about this one. They don't have much info for extras on their website.
I guess you have to contact them to register. I won't do it just yet - my availability is not great.

In general I wouldn't suggest for occasional extras to approach agencies. The only reason is, that most of serious agencies don't want 'weekend' extras on their books, but people who have great availability on day-to-day basis, often at short notice. Majority of serious productions happens during the week.
I am a 'weekend' extra, purely because I have a mortgage to pay. Simply cannot afford to leave my day job and hope to have a steady income from being an extra.
It is possible - I know that. I have met few people who are doing this full time, without having relevant education/training. I'm just not that brave.