moderated Re: Group "join" page not accessible when logged in
The key here (as per the subject line) is whether you are logged in or not.
Here's the corresponding URL in a group you're not a member of, so you can practice: https://groups.io/g/BowmanFamily/join <--note: not "joined"
Click on the above link, and you'll be redirected to your groups list. Rather mysterious, especially after the 510 bounce message you just received told you it would work.
Now log out of groups.io, and try again. Voila'!