Verizon is accepting Get My Data requests (a request to download your Group messages and files) until January 31st. The Archive Team would really appreciate it if you could do such a request, upload the data to an online file-hosting service, and send a link to archiveteamprivateyahoogroup@gmail.com
The Verge has published detailed instructions on How to download your Yahoo Group data.
They are looking for both fandom and non-fandom Groups where anyone could join (automatic approval, no moderator approval required). Any Group member can submit these Groups.