The Fediverse has an event organisation platform called @mobilizon, find out more at:
#Mobilizon servers let groups create events where participants can sign up, even without an account.
Mobilizon group accounts can be followed from Mastodon etc so people can see that group's new events appear in their home feed.
It's made by @Framasoft, the same people who make @peertube and provide all kinds of nice free open source online services.