This is going to be hard, but try to reduce their chances of "bypassing" Workfront. When we launch to a new business units, there are always people that will be like "Yeah, I'm having issues with this new system so can you just do this for me now?"
If you allow people to continue to use other means (email, Google Docs, etc.), they will and your adoptation of Workfront will be more difficult. I highly recommend having an executive send a communication with a clear cutoff date and make sure that executives in other departments (especially creative) talk to their people about the importance of receiving, updating, and completing things in Workfront.
It is hard to do because you don't want to mess with the flow of the business too much, but in the end it will be better.