How to add a member from an outside domain to Google groups
You must be an admin of your Google Apps domain to do this.
From your Gmail inbox, go to Groups.
Select My Groups.
Click Manage underneath the team.
On the left side, click Settings to pull down the list. Then click Identity. Make sure the Either display name or Google profile box is checked. Be sure to save this change.
Now, click Permissions to pull down the list. Then click Basic permissions. Make sure the Allow new users not in 'yourdomain.com' box is checked. Be sure to change this change.You should now be able to add users from any domain to this Google group.