Launch Contacts and select Edit > Edit Distribution List. In the sheet that appears select your group and you'll see your contacts along with their email addresses to the right. Simply click on the address that you want the group email to go to. When you next send a message to the group with Apple's Mail app, the message will go where you intended.
The distribution list isn't appropriate only for email addresses. You can also choose a default phone number and street address.
And that's it. Short and sweet--a simple way to ensure that your important group messages are sent to the most appropriate address.
Christopher Breen