Google Groups offers you an easy way to export and download your group's membership list.
1. To get into your group click on this link Google Groups (https://groups.google.com/) and click My Groups.
2. Click the name of your group.
3. Click Manage in the upper-right corner.
4. Check the box next to "Display name" in the upper-left corner to select all members.
5. Click Export Members near the top right corner of the page.
This will convert your list of members' email addresses into a .CSV file you can download and keep for your own records.