alt Hacker NewsWhich means it isn't free, it's a charity project paid for by the group organizer(s). That's probably fine in the short term, maybe even longer, but eventually the organizer(s) will move out of the area, or get tired of managing the group, or die, or whatever, and someone will need to manage it.
Honestly if it really would just need to cover the price of the cheapest hosting you can find and the domain registration a single small-to-medium Adsense ad in the sidebar might generate enough to cover it. I don't know how many impressions/pageviews it takes to generate $20 but it can't be that many.
I didn't say it would have to be free.
I run some meetup groups, and still pay for it for now, but the price hikes and other charges they're adding are onerous. I know many groups who have shutdown, and I know of some other players wanting to get in to this space to take over from meetup.com. $10/month to run a smallish group is affordable for a lot of folks. $50/month is not. $300/year is just not worth it to many folks, but... $99/year would be. Meetup seems to be moving in to some weird 'hyper optimize for short term revenue' move, which makes me think they'll be gone or acquired/dismantled in the next 5 years.