Adding external email addresses to an exchange Distribution Group

You need to create the external email addresses as Contacts in Active Directory. To do this, you'll need to leave the SBS wizards and use Active Directory Users and Computers.

1.) Open ADUC from Administrative Tools
2.)  Navigate to MyBusiness, right click and choose New -> Organizational Unit to create a new OU, and call it Contacts
3.)  Right Click on the new OU and select New -> Contact
4.)  Fill in the details for the contact, selecting an SMTP address - the SMTP address will be their external email address.
5.)  Now, you can add them to your distribution group(s)

Note: when you're adding them to the Distribution Group, you will need to explicitly include contact objects in the AD dialog box in the field "Select this object type".