Google has just launched its new cloud based service Google Drive to public, its in a roll out phase you might need to wait for sometime to get access to it.
But if you had got access and you are using it to sync your files on your computer then you will notice that it will sync all your Google Docs documents but it wont sync those documents which are shared with you by others.
This is done because by default you are not the owner of those files and they are not included on your My Drive folder, so if you wanna sync them with your computer then :-
Login to your Drive account,
On the left hand side pane you will see an option Shared With Me, click on it,
Now you will see all documents with are shared with you, just select the one which you wanna sync and then click on the option Add To My Drive,
That’s it now that document will appear in your My Drive folder too and after sometime on your computer as well.