If you attempt to send a message to a group that you're not [yet] subscribed to, an error message is returned. It says, in part:
>>> email@example.com (reading confirmation): 510 5.1.1 Your email address, youremail@..., is not subscribed to that group. To subscribe, send an email to firstname.lastname@example.org, or visit https://groups.io/g/groupname/join
Unfortunately, if you happen to be logged in when you click on the provided link, you don't land on the join page. Instead, you're re-directed to the list of groups you're already subscribed to (at https://groups.io/groups).
I don't know if this is a bug or a feature...perhaps someone here can clarify.