Editing external contacts from Outlook

We have a lot of external email addresses in our GAL. I need staff to be able to edit the contacts from their Outlook so they can populate the department field so I can then use that to organise the contacts into address lists. 

I have given myself read and write access on a contact held in the AD. In Outlook I pop up the Address Book. I search for the test contact. I right click it and select properties but despite giving myself read and write access in AD I cannot populate the fields. Is there a way to make contact properties editable through Outlook?

Thanks