On Yahoo it is possible to have more than one email connected to theCorrect.
In Groups.io each address is the login ID for its own account (not including Alias addresses, but those don't receive messages from any group).
Not sure it is necessary since signing up for an account is so simpleI would find it somewhat handy to be able to visit groups subscribed under different email addresses without the need to logout and back in; or to keep them in separate browsers/devices.
But that feature does add some complexity to the user interface, more choices for people to make, more ways to get confused. So any implementation would have to be very carefully designed so that it doesn't mess up the nicely streamlined sign up and join flow we have now.