google tries very hard to leave the impression that you must have a gmail/google/google+ (not sure, haven't had the misfortune of doing this for some time) account to subscribe.
http://groups.google.com/group/%(groupname)s/boxsubscribe?email=%(email)s
[1] https://github.com/bsandrow/google-group-joinUPDATE: wow, didn't know you can't even read messages without being google-user. Yeah, that's boorish a bit.
UPDATE2: turned out that's not true. You can read google group perfectly fine without signing in: https://groups.google.com/forum/#!forum/tmux-users