Situation slightly complicated, hope this helps:
There's @GetTogetherComm which is an open alternative to MeetUp.com. They aren't federating yet, but their first instance is up and running:
Then there's #Friendica, which is a FB-style social network which federates with Mastodon. It has events, but they only federate with other Friendica servers right now. There are lots of instances, for example: