There's no particular problem with sending emails that I'm aware of - though I do hear that sometimes emails from Voice end up in people's spam folders.
I don't expect it's anything you've done wrong. It could just be that the emails have been marked as spam by the receiving email server.
If you let me know the specific email addresses that you've invited to be admins (you can email at firstname.lastname@example.org rather than listing them on here) I can check to see if the emails have been sent.
Joe - Voice Admin