Former idiot / Co-founder of Letter.wiki
Thanks for your interest, Evan.
If you're referring to joining Fair Game go to the FB page click join, and answer a few questions. If you'd like to join Letter's public community group you can do so here.
Letter's writers group is private - if you write a letter in good faith you'll receive an invitation ;)
I'm one of the founders of a new online platform called Letter. I'm in the process of building a community around thoughtful conversation. We have a private Facebook group for our writers, and we host a meetup event (online) every few weeks. I'd like to welcome you and anyone else from the rationalist community to join.
Another fantastic community is Fair Game - a private Facebook group for respectful, "good faith" discussion.