When you get in, they give you three "guaranteed" interviews that you can schedule essentially any hour of the day with at least 24 hours notice. I suspect that they pay their interviewers for these interviews (hence the guaranteed nature of them), but don't quote me on that. After those three interviews, your available interview slots drop off dramatically; currently there is a two week wait period. If you do well enough (appears to be top 10%), they will start acting as a recruiter; you can have real tech screens through their platform anonymously, and "unmask" and go onsite if the screen goes well.
I'm not sure what the filter is for letting people in but I suspect it's fairly manual right now, especially if they're paying interviewers for the three guaranteed slots people get. Keep in mind that Gainlo charges $100+ per interview for the same service. I actually got better feedback from interviewers on interviewing.io than the one interview I did from Gainlo (YMMV of course).
Google remote onsite on Friday!