One of the pieces that makes MeetUp.com so valuable is its centralized nature as a one-stop-shop for every locale. To better disrupt the space and still provide that value, perhaps an open source version that could be very IndieWeb friendly might attempt to act as an aggregation hub and provide some of the services while still allowing people to post their events and RSVPs to their own sites, but still provide the clearing house to bigger communities. This could be a service like meetup.com or upcoming.org but it would work more like IndieWeb News or Kicks Condor’s IndieWeb.xyz.
The resulting workflow would look like roughly like this:
An organizer posts an event with details to their website and then uses webmention to syndicate a copy to the hub event site. The hub then parses the basic data and allows it to be displayed on pages that were sortable by date and city (at a minimum). People could then have their one-stop centralized location for events, but then RSVP directly to the original on their own sites as you indicated.
To take things further, additional useful services could be added by the hub in the form of a micropub client that organizers could use to input all their event data and then publish it to their micropub capable website. (Quill already has the ability to post events like this as an example in the wild.) Similarly, for individual attendees, the hub could have a micropub client to do RSVPs to both the attendees’ websites (and/or the events’ site) as well.
Naturally this presupposes that a benevolent actor(s) could serve as the hub and handle the maintenance and overhead of that piece.