How else could they join if they are joining out of the blue?
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If they know the group's email address then they can use the +subscribe email command. They could learn that if, say, a member forwarded a message to them or you publicize that address at other sites or publications.
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In Ed's case he said he publicizes the group's +owner address in a newsletter so that he can send instructions on joining by email.
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But I've let my curiosity wander far enough off Ed's original question. Alas, the answer was "no" (but it is a good suggestion).