[abaines]

You can subscribe via email without any google account by sending a message to <groupname>+subscribe @ googlegroups.com, then it acts just like any other mailing list.

[pyre]

This[1] also used to work, but it's been a while since I've used it. Basically just hits this url:

  http://groups.google.com/group/%(groupname)s/boxsubscribe?email=%(email)s

[1] https://github.com/bsandrow/google-group-join