There has been much discussion which I won't repeat.  Many commented that, for certain users, getting them to click a link in an email is a challenge (and I understand that).
I have 2 suggestions:

1. Provide a means for owner/mods to resend the "re-subscribe" email with the link:
   1. with updated easier to understand language and terminology
   2. allow a customizable section for a personalized message
   3. the From: address is the owner/mod, not the group address
2. Provide a means to send a "re-subscribe" email - but this email does not contain a link:
   1. allow a customizable section for a personalized message
   2. the From: address is the owner/mod, not the group address
   3. the Reply-to: address is set to a special address which will resubscribe the member without further ado (perhaps GROUPNAME+resubscribe@groups.io).

Suggestion 1 has been mentioned elsewhere along with extending the time period for the Re-sub link.
Suggestion 2 does not require the member to go to any web page, they stay in their email.  They only need to click Reply and Send.  I would suggest that the resubscribe command email does not generate the welcome message or an email containing a link to confirm - if the sender was previously subscribed to the group.  This would require groups.io to retain knowledge that the member was unsubscribed from the group, essentially in suspended state as has also been suggested.

Thanks
Toby

How is this functionally different from an invitation to the group?
--
J

Messages are the sole opinion of the author, especially the fishy ones.
My humanity is bound up in yours, for we can only be human together. - Desmond Tutu

The verbiage would be different and, in the member's mind, they may not even know they got unsubscribed so would confuse them to get an invite to a group they are already in. 
Also, no subsequent welcome message.
But could coalesce both suggestions into single email - either link or reply would resubscribe.
Was thinking that having From: be the owner/mod sender would avoid message going into spam folder.
Another thought - email containing many links is sometimes considered "spammy" so email with no links might be better.
Toby

Toby,

3. the From: address is the owner/mod, not the group address

I think this is a bad idea (unless you mean the group's +owner address).

The problem is that the receiving service will look at who delivered the message (Groups.io) and who is named in the From (somemod @ example.com) and treat the message as having been "spoofed". This will probably make the message more likely to be delivered to the spam folder, the opposite of your intent.

A message that is honestly from Groups.io, or better still a message direct from the mod/owner's email service, are almost always more likely to be delivered to a person's Inbox than a spoofed message.

Suggestion 2 does not require the member to go to any web page, they
stay in their email. They only need to click Reply and Send.

I like this idea. Along with your later suggestion that 1 & 2 could be combined: a single message that lets the (former) member choose whether to reply by email or click the link.

I would suggest that the resubscribe command email does not generate
the welcome message or an email containing a link to confirm - if the
sender was previously subscribed to the group.

This is already true of the notice containing the resume link. So that should be easy.

This would require groups.io to retain knowledge that the member was
unsubscribed from the group,

This also must already be true for the resume link to work.

Shal

Mark,

I wrote:

The problem is that the receiving service will look at who delivered
the message (Groups.io) and who is named in the From (somemod @
example.com) and treat the message as having been "spoofed". This will
probably make the message more likely to be delivered to the spam
folder, ...

It occurs to me that this also applies to normal group postings coming from services that do not require the DMARC mung of their From address.

So maybe that workaround on the outbound would help. That is, for every /receiving/ service that has a high rate of unsubscribes, mung all From addresses sent to that service.

I noticed that it was generally the same services that publish DMARC p=reject that show up as those operating FBLs with the highest perceived rates of member unsubscriptions. Maybe that's not a coincidence, maybe those services are also particularly sensitive to inbound spoofs, even when the sending service does not publish a sequester policy, or has no policy at all.

Maybe.

Shal