The first time I tried to invite to join my YG I had the same issue. Long story short, I believe the problem was a typo on my part. I may well have sent the first invite to groups.i.o. (note the extra period). I suspected, and Don strongly suggested, that if the transfer status bar on the page didn't acknowledge receipt of my invitation then they probably didn't get it. So, I sent a second invitation, taking extra care with the address, and the transfer status bar on the .io page very quickly changed to a confirmation that my invitation had been received.?