If you have the message archive enabled you can export your group's entire message history. Messages are exported as a .mbox file.
On your group's Settings page, there is an Export Messages section where you can request an export of your messages.
For groups which have been around for a while and have lots of messages, the export process can take a long time (upwards of 6 hours). Refresh the Export Messages table to see check the progress of your export, however, once the export is completed we will email you.
When the export is finished you will be able to download the .mbox file from the Export Messages table. We recommend you delete the export from the list as soon as you've downloaded it. This is because the archive files are accessible to anyone with the url.