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