Replying to an email address to subscribe is a really nice solution.That's what the Invite mechanism does: sends the person an email. Their reply to that message adds them to the group's mailing list. Providing the person with the group's +subscribe address might be effectively the same thing. Hm, I guess I should test that.
Why does each member need to authorise the transfer of theirBy international copyright convention each author has an implicit copyright to their writings. Most authorities I've read say this extends even to such small works as message postings in online services. It certainly applies to photographs and other images.
But beyond that, it is just good business (I think) to show that level of respect for your users and their content.
In the case of Yahoo Groups, Yahoo asserts that the contributing member retains ownership of the content - that's a promise that the group moderator must keep, or else be in violation of Yahoo's Terms of Service and Community Guidelines.
What the difference between moving mailing list providers and movingThe difference is that the individual members have a contract with Yahoo (or, presumably, Google equivalently) regarding the disposition of any content they contribute to a group. It is not the group manager's content to relocate without permission.
In a pure mailing list (absent message archives and other stored content) this would not be a problem - the members must agree with the new list provider's terms of service, but that's usually treated as implicit in their use of the service.