Written with a view to meeting the following criteria:
"You: enjoy careful thinking and value good communication."
Don't. At least outside of Silicon Valley where oversubscription may actually be a problem. It's a good intention but it inevitably will make people worry they aren't welcome or aren't the right sort of people . Instead, describe what one does or what one talks about in a way that will appeal to the kind of people who would enjoy coming
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.
For the London meetups I write this:
We're a fortnightly London-based meetup for members of the rationalist diaspora. The diaspora includes, but is not limited to, LessWrong, Slate Star Codex, rationalist tumblrsphere, and parts of the Effective Altruism movement.
You don't have to identify as a rationalist to attend: basically, if you think we seem like interesting people you'd like to hang out with, welcome! You are invited. You do not need to think you are clever enough, or interesting enough, or similar enough to the rest of us, to attend. You are invited.
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.
As part of announcing meetups publically, it's good to write in the meetup description about what kind of people would likely be a good match for the meetup. I still haven't gotten a good description myself.
How would you describe the kind of people we are in words that are clear to outsiders?