this io.groups
site should really be looked into. This site breaks many of the
"Terms of Service" conditions.
Per the TOS page (
/static/tos)
you can report a group for the specific TOS violations.
It was the group
owner who did the posting. And not the first time.
And while at it, there's nothing stopping you from also including
the additional info about the slanderous posting in that email
you'll be sending.
Cheers,
Christos