I have the same issue. I have tried to add users but they never receive an invite. I have tried to resend them multiple times and gone through the usual process of making sure its not hung up on the server in a spam folder.  It is unfortunate. I certainly can not use a service that I can not invite clients to review and see our bug testing. I'm literally trying to train some today, but now I have to let them know that I am looking for an alternative solution. I know this service is offered for free, but I would of gladly paid for a functional product. This was project one for me, so the justification of increased spending on this is looking not so good. Its time to find the market places competitor for this service. Its sad because I was excited about using this in my project work flow.