Is there any way a Yahoo group could be copied to Groups.io without thatI assume in this context that none of those moderators has the privilege to "Add, remove, and change moderator privileges" (because if any of them did, they could use Easy Group Transfer and needn't ask for your help). So presumably they've extracted a list of the members' email addresses by some other means.
If none of those moderators have access to the members list (possible, but I'd imagine unlikely) then there's nothing much you _can_ do for them.
Alternatively, could I and the other moderators of this Yahoo groupIf they're willing to pay the fee for Direct Add (and have extracted the list) then again they don't need your help. If not, they can use the Invite page.
So I'll agree with Trish, Judy F, James, and Dano (and you): in general no aid is needed. I suppose one thing they might be asking is for you to waive the Direct Add fee for their situation; I wouldn't - importing the complete group has to be worth $10 to them. On the other hand, I don't think you need to do anything to hinder their efforts using Direct Add or Invite.
That leaves the messages in question.
Again they only need your help if they can't use Easy Group Transfer.
In that case they have to download the messages themselves (using the Chrome Application). You've already answered for yourself that copying the messages doesn't present a legal problem for Groups.io, so I don't see a strong reason not to help them. Except that it is a manual effort for you, which doesn't scale. So it is up to how generous you're feeling.