Well there is no screening whatsoever, so basically anybody who feels like being a model can just say they are one. Do not expect anybody to have any experience at all unless their portfolio proves that they have it.
Also, you'll get a lot of unprofessional people from there if you're not careful. As in, a good portion of the models (and probably photographers too, but I don't have any experience with them) treat it more like a dating service and might attempt to hit on you and/or sleep with you, etc. Somewhat less of an issue if you only want to shoot normal fully clothed shoots and not nudes or boudoir or fitness, etc.
If you screen the models carefully though, you can get some really good people to work with. Look for:
1) Professional looking portfolios. Even if they don't have much experience, you want to still get a sense for whether their portfolio looks like they care about TRYING to achieve art and real modeling, or if they look like "oh hey my ex boyfriend had a nice camera and took some racy photos of me with a dirty lens on a whim" sort of shots.
2) How cogently they have written their profile. Legit models will normally have a specific list of kinds of shoots they're excited about, will have reasonable sounding professional-style policies listed, will explain about themselves more, use proper grammar, etc.
3) Professional sounding correspondence if/when you contact them.