The most generic solution which should work for every address/mail provider and every Google group is send a mail to
Some Groups seem to be restricted to mails registered to the Google account. An example is the orient-database group - visiting
won't let you choose an arbitrary address. I could only add one alternative address besides my Gmail address, so adding addresses to the Google account isn't a solution.
EDIT 1: The secret is that one can subscribe to any google mailing address with an address unknown to google when logged out (the first time you log out, google logs you in, just be persistence - just like when you'd remove a virus from your system ;))