How to default the picklist value group field based on the user?

I have a set of picklist fields where we need to permit a set of user roles (Sales) to select a subset of values.  There is another set of users (Marketing) who need to select a different subset of values.  Some values are available to both Sales and Marketing.  How can I set the Picklist Value Group field based on the current user?  We can add something to the User record, but what workflow trigger would be used to set the PVG field?