"... Nope. They will get an e-mail or SMS or gtalk message, and they would reply simply by replying to that e-mail/SMS/gtalk bot. Your friends don't have to sign up. ..."
Ok, that solves the signup problem.
So you still need to get the contact details. Not as hard as getting friends to sign up. What is the advantage of non group creators (non-admins) signing up then?