If you haven’t contributed to WordPress before, take a look at this great slide deck from Petya and follow these directions to get set up with your WordPress.org and WordPress Slack accounts.
Here’s a way to contribute on Contributor Day and beyond: go to the new Gutenberg Test Site, create an account, and try out the Gutenberg Editor. If you test the site, I’ll give you a Gutenberg Tester sticker at the Contributor Day meetup.
Our May 2017 meetup discussion ranged from choosing a theme to WooCommerce marketplaces to why a high PageSpeed Insights score may not be worth pursuing.
Want to learn more about WordPress Multisite? This list of how-to articles and useful plugins will get you going.
Here’s a bibliography for the April 2017 meetup presentation about Flexbox and CSS Grid. More is being published on both of these topics all the time. You should definitely follow Rachel Andrew to keep up with Grid developments.