Registration Problems with Google Groups

I set up a Google Group to communicate with the contributors to the book on evidence-based software engineering that I'm pulling together for O'Reilly, but have run into some registration snags. I invited everyone into the group using their regular email addresses, but Google isn't letting some of them confirm (which means they don't get email sent to the group). They can confirm when I invite them via a GMail address, but they don't read that regularly, and anyway, this stuff is just supposed to work, right? Has anyone run into this before? If so, what's the cure?