Yeah, there's a couple:
@GetTogetherComm is a free open alternative to meetup.com:
(It's not federated yet, but that's on the roadmap. For the moment you can use this instance.)
The other option is Friendica, which has a Facebook-style events calendar feature.
Friendica federates through ActivityPub so it's compatible with Mastodon, Pleroma, PixelFed, PeerTube etc. https://libranet.de and https://social.isurf.ca are pretty good instances.