I've noticed that there is a set of links available at the bottom of Individual messages from Groups.io, that do not appear at the bottom of Digest messages.
The Individual messages have the standard reply options (View/Reply online, Reply to Group, Reply to Sender, Mute this Topic, New Topic). Then below that, there are the additional options (Change Subscription, Group Home, Contact Group Owner, Terms of Service, Unsubscribe).
When messages are received in Digest form, there are the standard reply options listed under each message. But the additional options available in Individual form, are not shown below the final message in the Digest. There does appear in the header of the Digest, the link to Change Subscription. And in the footer, a link to Unsubscribe. Shal pointed out that the group email address also appears in the header and footer, but I think it is unclear that it is not a part of the Change Subscription or Unsubscribe links. I would be hesitant to click on the email address, at the risk of unsubscribing.
I suggest that the additional link options should be added to the bottom of the Digest message in the same format as they appear in the Individual messages. I would particularly like having a Group Home link. Perhaps leaving the group email address in the Digest header would be handy too, as some subscribers might not scroll all the way to the bottom of the digest.