I've been thinking about something like this lately and here it is! Mobilizon promises to be a good, free and open source, federated alternative to Meetup and Facebook Events. Join their crowdfunding campaign! https://joinmobilizon.org/en/
This Mastodon instance is provided gratis by the NZ Open Source Society for the benefit of everyone interested in their own freedom and sharing with others. Hosting is generously provided by Catalyst Cloud right here in Aotearoa New Zealand.