The current process is actually incomplete.
Groups.io attempts to send a message to the person whose service provider or themselves, marked a message as spam. This message has a link to enable the member to automatically rejoin their group(s?).
Groups.io also sends a message on the Group Owner channel (to Owners and all eligible Moderators) that this action has happened. This message does not have the link for the user!
BUT, in many cases, the message is never actually seen by the targeted person because, well, output from groups.io is permanently marked as spam!
Owners could reclaim many of these people if you were to put the specific link for the unsubscribed person into the message to the owner.
We could then use our own email which is not marked as spam to inform the member in time to recover their account.
Most members are not rocket scientists and my latest to run afoul of this was removed for marking spam on May 21 and didn't notice it until last week even though they are on Daily Digest, duh!