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Ken,

I am trying to help someone join my Group, but they have a BT email
address and so any attempt to join by email simply results in the
subscribe email being sent to the main Group address (as BT strips out
the +subscribe part) and is of course rejected.

As Chris said, that won't happen if they use their webmail interface rather than an SMTP client.

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 ...

That message says that he needs to use the Log In page, not the Sign Up page. He should believe what it is telling him.
/g/GroupManagersForum/wiki/Logging-In

which is clearly impossible as he is NOT a member, so his address is
NOT registered ...

Your inference is incorrect. A membership (subscription) in a group is separate from whether the person's email address has been registered at Groups.io. A single address (account) can participate in multiple groups. And even if this person is not yet a member of any other groups, the account may exist as a result of prior attempts at joining a group.

and there’s no way he could ever have logged in previously and created
a password.

Again incorrect. You, as a group owner, cannot tell if they have or not - all you know is whether they've joined your group. The account exists independent of any group memberships, and even independent of whether the person ever logs in to use it.

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

The person needs to use the Log In page. On that page select either the "Forgot your password, or don't have one yet?" link or the "Email Link To Log In" button - both do the same thing.

Once logged in to Groups.io, the person should be able to Join your group (or Request membership, if your group is restricted).

Shal


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