@turiphro: allowing a customized email might work, but I personally would still prefer just being able to redirect the "Join" button to something other than the default behavior, because I can just slap a URL there and be done with it. I do not want to be in control of who joins: everyone that wants to join should be able to sign up and immediately get in the conversation. Instant gratification. If you need to wait for a moderator, the excitement might be lost.
@Arjan: for us, what is working really well is a twitter account + Slack instance (adding slack-in to allow people to invite themselves). I personally haven't had the need for a "corporate email address" of sorts.