FROM THE ACCOUNT YOU USE TO RECEIVE MAIL FROM THE LIST, send an email to weewx-user+...@googlegroups.com. You can send the request with no subject or body text.
If you tried to unsubscribe and are still getting email, make sure:
You unsubscribed using the same email address that gets the group email.
The email is not being forwarded from an address that still belongs to the group. If they are, email the address and ask them to remove your address.
You’re not signed up for the group with more than one email address. If you are, unsubscribe that email address as well.
You’re on the desktop version of Google Groups. You can only leave a group from the desktop site.