Thanks for your comments. I may be confused as well!
Your are right about the Login & Join Website buttons appearing if you are not logged in (unless the group is private when only the Login button appears in the page footer).
The documentation mentions a "Join Group" button (see https://cambridgeopensystems.com/documentation/community/administering/users/) which I'd hoped streamlined the joining process, but I suspect it is just that the button is actually called "Join Website" not "Join Group".
Thanks for again,