Mobilizon is an event platform for the Fediverse which lets people and groups create events pages where participants can sign up, even if they don’t have an account. There’s lots more info on the official Mobilizon website ⧉.
Organisers can create a Mobilizon account on an existing server, or even start their own server if they prefer. Once an account is created, the organisers can use it to post event info pages where participants can say whether they are attending.
Mobilizon accounts can be followed from Mastodon etc, and when new events are added these will appear in followers’ timelines. They can then click through to the info page where they can find out more and say if they are interested.