This is just the latest in a long line of hurdles trying to get an app ready for the Apple app store. Some of my colleagues are able to accept the invite and for others they just get a spinner or a message saying the invite is expired, despite having only just invited them. I've also tried re-inviting and completely deleting and re-adding the user. Feels to me that the load balancing dishes out good servers to some and bad ones hosting a broken site to others.