If you don't mind using #bash, you may try a tool I wrote
https://github.com/icy/google-group-crawler
It can download all mbox files from Google Group. If you have a cookie file, you can even download all files from a private Google Group, and/or to see all original emails. It can also read rss feeds and fetch the latest posts ; and this is useful for daily mirror.
An example result is here http://l.archlinuxvn.org/archlinuxvn/. MHonArch is used to convert mbox
files into HTML format.