First open the Address Book in outlook from the menu bar at the top.
Click on the file menu at the top and select "New Entry"
Select "New Contact Group" and click Okay.
At the top of the new group window, type a name for your custom contact list.
Click the "Add Members" button and select "From Address Book"
Search for and add as many users as you would like to your custom contact list.
Click the members button at the bottom and click Okay.
Once you are happy with all of the members added to the group click Save & Close.
The new custom list is complete. You can add it into any e-mail now from inside your personal Address Book.