I agree with John, with a couple of additions.
As he said, the wording of the default message is misleading/wrong and should be corrected. Replying to the message does not activate the account if it is a restricted group. This is misleading the user, and likely at a time when they are new to Groups.io, leaving them asking why their membership did not get activated. To me this is not something that is a “let’s consider for the next release,” but an error which should be fixed.
Additionally, if they click on the “Confirm account” link, it can be even more confusing because they are taken to the confirmed page, but they still need to be approved if it is a restricted group, and there is nothing on that page about the approval process. Then if you click on the group you just joined, it takes you to their “home” page, but still says nothing about the approval process. Only when you click on one of the side tabs do you get the “Your subscription has not yet been approved.” message. There should be a separate page for public and restricted groups.
For what it is worth,