Firstly, you will need to create a track, do this by selecting "Course Maker" from the main menu. Select any theme and then simply select "Save As" from the bottom left of the screen. I did not make any changes to the track and the trophy unlocked.
Next, to share the car go back to the main menu and select "GT Mode". On the left hand side of the screen there will be 8 round menu options, select "Track List" which is on the bottom row, right hand column. The track you just created should be in the list, press triangle on it to bring up the menu. Scroll down to "Share With Friends" and make sure it is set to "On".
The trophy should unlock within a couple of seconds of turning sharing on.