Select the gear icon in the top-right of your Facebook Group and select Edit Group Settings.
In the Group Address section, click the Set Up Group Address button.
Enter a name in the "[name]@groups.facebook.com" box. In addition to creating an email address that can be used to email all of your group members at once, the name you enter into this box will allow users to gain quick access to your Facebook Group. For example entering:
email@example.com .. will allow users to access the group by going to: