I would strongly advise to stay away from building a community on slack. It's not the best platform to build a free and open privacy conscious community. Semaphor by SpiderOak is a better option in this point in the project's reboot. It's free and requires no e-mail or password to sign up, there is a recovery sentence associated with your account. The company behind it was endorsed not once, but twice by Edward Snowden.

"SpiderOak doesn't know your information. That's what Zero Knowledge is all about. Everything is encrypted. There is no "reset your password" with them because by doing so that would negate the encryption. So, make sure to remember your password phrase. Believe me, this is a good thing."

http://www.inc.com/travis-wright/a-secure-slack-alternative-that-edward-snowden-would-love.html