If I am playing a game through game share can I still download and upload to the workshop

My friend owns cities skylines which is a great game so I decided to game share it rather than torrent it as I wanted access the workshop. However, when I go into the workshop, the subscribe button is greyed out. Does this mean I have to buy the game or was steam just playing up?

EDIT: I can now subscribe to and download items from the workshop fine. However, when trying to upload to the workshop i get the following error:

Error submitting workshop item:Fail

[Workshop exception]

Details: No Details