In kinda answer to your question;

Background: I run a lesswrong group - we are not on meetup. I have run other groups in the past and attend several groups as well.

From another LW group; and something I take to be true; (from the inside of a group) meetup seems to cross promote your group to many many people; whether they are relevant people or not; is not up to meetup to deal with. They then show up; and realise that they are not quite the target; then leave. In the mean-time your group gets diluted while the meetup-style deluge of people appear and disperse.

<them people I try to avoid> happen less often than I have expected. From personal experience; trying out a group once does not equal permanent commitment; its worth trying once; if only to see who the <losers> are in your town and decide it is in fact worth avoiding them.

From a cost/benefit or utilons perspective; you are burning a few hours for a range of opportunities including meeting new people; finding new good things and learning new things.