Trying to avoid the email route I asked him to try and subscribe via the website, but when he enters his email address (required to confirm) there is another error, stating that email address is already registered and he needs to log in which is clearly impossible as he is NOT a member, so his address is NOT registered and there’s no way he could ever have logged in previously and created a password.

My guess is that the apparent attempts to post have been logged by the GIO system and that email address is now flagged as restricted in some way and so prevents him from joining. I have confirmed this error claiming his address is already registered, but I can find no mention of it anywhere in my Group - apart from the apparent posting attempts in Activity. There is no mention anywhere of that address being registered in any way. So any restriction is obviously hidden from even the Group owner.

Can anyone suggest how to clear this restriction or otherwise explain what is going on and how to actually allow this person to join?