The most simple way to do this is by creating a mail enabled group in Active Directory. I don't have a SBS server in front of me at the moment, but you should be able to do this from the SBS management tool.
Once you create the group you mail enable it and then add the user accounts into the group. If the email addresses are not in Active Directory (or not on your Exchange Server) you need to create contacts in AD and then add those contacts to the group.
Let me know if that makes sense to you.
- Joel