Some of us in the IndieWeb space have been posting events and details on our own websites while others are posting RSVP replies on our  personal websites and sending webmentions to the event posts.
Here’s an example of an event: https://2019.indieweb.org/summit and my RSVP to it: https://boffosocko.com/2019/05/30/55753006/ using this sort of set up.
There’s also a self-hostable service called Meetable that is IndieWeb friendly and accepts RSVP webmentions as well. If you’d like to test it out with your new webmention functionality, you can RSVP to one of the upcoming events at https://events.indieweb.org/. We’d love to have you join us if your schedule allows.
Other Meetup.com alternatives we’ve documented: https://indieweb.org/meetup.com#Alternatives