Blockchain doesn't offer significant advantages over a (much more simple) ActivityPub based event planner. We do not need a signed chain of hashes to track that a person is coming to an event, but rather a simple RSVP.
If the event planner is a single centralized global entity, then the end-user features are less significant, that is correct - well aside from no one being able to take down the data and it being open source. If you are talking about everyone hosting their own, then this is incorrect - even if you think they could be federated "discovery" of new groups and members is practically impossible when a new server is added. There is a significant advantage of having a global registry, when it comes to discovery.