In the very simplest case, it could be one profile per meetup. The profile creates nip-52 calendar events, that's the linking between the meetup and the organized events.

Some other rules:
#meetup tag is mandatory in bio.
Organizer npub must be specified as a profile field

Then all we need is a client that fetches #meetup profiles from relays and displays them and their calendar events.

Just an idea. Not sure if we need more features than this.