How might we implement this if we wish to do it? I think when you sign up, there is no post sign up message or screen, and I don't believe you can prescribe or suggest communities without home brewing the solution.
Email is optional, so we can't rely on a post sign up / approval email to catch everyone.
It could be included in the sign-up page its self.
Or it could be included in the sidebar of the site too.
Just some thoughts, not a bad idea IMO.
Edit: A post approval PM with some welcome information that includes a link to the introduction comm would also work.