Are these by invitation only, or did I miss out on the opportunity?

You can join one immediately. You didn't miss out, there is still space.

Under "Schedule"? It's saying "event full" for all of them. Sorry for putting this in "suggestions" instead of "technical help," by the way.

Week 4, scroll to the bottom you'll see an open one.

EDIT: Unless you are already confirmed at 5 other events that week.