Group Owners and Admins have the ability to edit the short and long descriptions of their group.
Here's how to change the description:
1. While in the group, click on 'More Admin Tools.'
2. Click the "General Settings" link from the dropdown menu.
3. Scroll down on the page until you see "Short Description" and "Long Description". The "Short Description" is what will be seen on pages where your group is listed with other groups. The "Long Description" is what will be seen on your group's homepage.
4. Be sure to click the "Save" button at the bottom of the page when you're done editing your group settings.