small data point from here, when I stumbled upong meetup, I was doing search engine lookups for things like '<activity group> <mycity>' - and found meetup to have what I was looking for and more - I enjoyed browsing it, using it, and appreciate the email reminders.
I think a wordpress plugin would be fine for getting much of the network effect if it had title tags (with city perhaps) and everything SEO right so events showed up in search results fine - a bonus if it could include a 'link wheel' I think it used to be called(?) in the footer showing other meets in same city - perhaps pulled from an API with this thing coming from freecodecamp - or some other apps that could do a same scheme or something.
I noticed buddypress (plugin for WP that adds groups and such) is now API friendly - so I would think these things could be mashed up pretty easy and quickly these days.
I really appreciate(d) what meetup has done for me in the past and I'd like to see that kind of thing continue to be an option for future humans and not have the burden for fees so it's only pay for play.