Some discussion has occurred in our Groups.io discussion group that needs to be removed from our Groups.io discussion group and transferred back to our Yahoo executive session group for legal reasons. Does anyone have a clue if that's possible and how I would accomplish it?
What do you mean by "back"? If all of the messages in question were originally on the Y!Group then they should still be there - the transfer agent copies content, it does not alter the Y!Group in any way.
If some or all of the messages were posted only to the Groups.io group then you're out of luck - there is no such mechanism, and to create one Yahoo would have to build it.
The best you can do is forward them to your Y!Group from your email, or if you only have them in the Groups.io group's Messages you'd have to copy/paste the content into a posting for your Y!Group. And afterward delete them from the Groups.io group.
One concern I have is that you mention "for legal reasons". If there's any chance that rules of evidence apply to these messages then the techniques I suggested will not preserve the chain of custody information contained in the header of the messages. To preserve that you'd have to copy/paste the "source" or "original" of the messages, and then be able to vouch for the authenticity of your copy. Otherwise once you remove it from the Groups.io group a legal adversary could claim that you may have altered the message.
More Help: https://groups.io/g/GroupManagersForum/wiki
Even More Help: Search button at the top of Messages list