it's just an online forum, so "vouching" is not a particularly big deal. So if someone is vaguely aware of you from somewhere else online they'll probably give you an invite, or even if they've never heard of you but you have some kind of public footprint they can look at. Or if something you've built or written ended up on the site, that's pretty much an automatic invite since people should be able to participate if something made by them is discussed.