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.
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.
Want to know more about web accessibility? Here are some resources.
Find the WordPress site management tools we discussed at the February 2016 meetup here.