Shal wrote in post #9840>>Quoting David Neither do I see a way to perform a bulk deletion from a groups.io group. Shal replied: Nope. Here's another person with the same request:
https://groups.io/g/beta/thread/494463<<

There is one idea I have that might bypass that issue. If you had the entire message archive transferred over here to groups.io and put into a subgroup, read only, then they could save their own messages that they want from it, and you could have a deadline to get them all, and once everyone has what they wanted, you can have the subgroup deleted.

Hope this helps,

Brenda