If you'd like to export your list of members' email addresses into a backup file, or transfer your Google Group membership elsewhere, Groups offers an easy way to list these email addresses in a single file.
From your group's homepage,
- Select Manage in the top-right corner.
- Select All members in the left sidebar, then
- Select the Export members button.
This will convert your list of members' email addresses into a .CSV file you can download and keep for your own records
Reference: Google Groups Help - How to export your group's member list
Unfortunately, they do not yet have the capability of doing a domain wide export for all groups. I am hoping to submit a feature request about this today.
I have tried that method. we have 2 google groups. i have successfully export all emails by followed the above method. But in another group when i click the export list button it will show like this:- "members list couldn't exported". i need to export it also how can i do it. is there any other method to download all emails in google group