I help run a meetup group unrelated to FIRE, and my advice is to understand your competition. Your competition is the phone sitting in your pocket and the massive TV screen in your living room. To the extent our generation is devoting hours per day to electronic entertainment, they are NOT getting involved with social clubs, civic groups, self-improvement groups, or political groups. As these groups have atrophied over the past several decades, life has shifted to a lot less making friends and a lot more staring into screens. The phenomenon was documented as early as 1999 by Robert Putnam in
Bowling Alone.
The results:

The thing to understand about your competition is that they can generate the illusion of interpersonal relationships while supplying intermittent doses of dopamine to the users. Your meetup can supply genuine relationships but will struggle in the dopamine department. You have the higher-quality product, but it's like competing with McDonald's by selling fresh fruits and vegetables.
The thing to understand about your target audience is that they are miserable, and yet they keep going back to TikTok, Facebook, Twitter, and Netflix in an addiction cycle. You're probably looking for the people who are ready to quit the entire apparatus and looking to rebuild a real-world life. I would caution against the idea of building too much of a social media presence around your group, because the social media services will simply distract your audience away. Get a meetup.com account instead. Likewise, I would caution against any meetup activity that involves watching a screen together, because this just reminds people how much dopamine they could be getting watching their own screens alone.
Some of the most active real-world groups in my town are bicycling clubs and running clubs. These mix reality-based friendships with an alternative source of dopamine in a context that pulls people away from their screens. If you could get involved with some of these groups, you could handpick a few people to invite to your radical-transparency mutual-accountability FIRE club.