What is the error reported when you try the following?
set DocBid.Author = DocBid.Creator
I would not expect this to work without using the set operator. set is not a Formativ command, rather it is a VBScript operator to be used when assigning an object reference to a variable. In this case, you are assigning to an object property (Document.Author) which is possible only if it allows write access in the installed version of GroupWise.
Have you tried calling the following method before assigning Creator to Author?
I hope this helps.