I'm in the process of developing a Google apps migration/archive system and at this point in development I'm trying to come up with a way to download all messages in all the groups that my domain users have created. I know that I can set up forwarding filters and have all messages archived to an email, but this doesn't help with older messages.
Is there a way to download these messages from a Google group and if so, is there away in the admin API to get a list of all groups that users have created?
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.