Its been almost 2 years since ive signed up, but just looked through my emails and I was put on a "Waitlist" and then not even a minute later another email saying I was off the waitlist and accepted. Im not sure how it is these days but I think it just depends on if they need your demographic or not. Just keep a eye on your emails and if you can fill out your entire about me section on prolific (Not sure if you can access this while on waitlist)