Create the external contact
Open Exchange Management Console.
Under Recipient Configuration, right click on Mail Contact and select ‘New Mail Contact…’
Select ‘New Contact’ unless you have already created one in Active Directory and want to link it, you would then choose ‘Existing Contact’
Enter all details as required. Click on Next > New > Finish.
Open the properties of the contact. On the general tab, tick ‘Hide from Exchange address lists’
In Active Directory Users and Computers*, open the properties of the Distribution group you want to add the contact to
On members tab, click Add
Click Object types and ensure that Contacts is selected
You should now be able to find the contact and add them to the Distribution group.
* Note: you can also add the contact to the Distribution group in Exchange Management Console rather then Active Directory Users and Computers