I now have three contacts with the same problem. The third may have given a clue: he is trying to use this group via the old Yahoo group. I don't know if that group even exists any more or how much access it gives to Groups IO. I will look into this further with them. I think the simplest answer is for them to start again with this group and forget about Yahoo.
However, this does not explain why I got no reply when I emailed our Group Owner.
