The @GetTogetherComm project is making an open source federated alternative to MeetUp.com.
You can already use their first instance (https://gettogether.community) but they need help with building the federation side of things, so that instances can link together and with the rest of the Fediverse.
If you're a coder and interested in helping, you may want to take a look at this post:
mastodon.at is open to all users and federates with most instances.
🇩🇪 🇦🇹 🇨🇭 mastodon.at ist offen für alle User und ist mit vielen anderen Instanzen verbunden.