Not sure what your meetup content is, or how you feel the real criteria for someone fitting in are. Are you going to talk about science, or technology, or philosophy, or are you going to to do some kind of exercise or group activity, or are you just going to hang out?
For meetups I've run in the past, I think the most important criterion of fit is that someone should enjoy training their cognitive skills (which was usually the meat of the meetups), and enjoyment of LW subculture ("did you see X?" being a good way to have a fun conversation / hang out) was an important secondary quality.
Oversubscription isn't an issue but that doesn't mean that it doesn't matter who comes. If a lot of people come who aren't a good match they water down the meetup and the regulars don't come anymore to events with an open invitation list but prefer to come to events whose attendence is more selective.