You can join irrespsective of where your site is hosted or what technologies you use! We have people who use static sites (like me!), WordPress, dynamic sites, micro.blog, and so much more in the community.
I'm not sure why the Brighton one has that link. I'll take a look. The challenge you mentioned -- build (or add to) a site in an hour -- will be hosted in two weeks (see events.indieweb.org for more); last time we had someone make a bookmarks page, a PNG header reader, a game with computer vision (this one was me!), a site on the Gemini protocol, and more.
Etherpad is where we take notes during sessions. It is a collaborative, open source editor.
I will check out the London Meetup this wed. Brighton one links to this Wordpress “challenge” https://60minute.site/ and something about etherpad https://etherpad.org/
Finally find this obvious link https://indieweb.org/Getting_Started