You can show all the bookings associated with a specific key onto a user's diary in the following way:
Open the planner containing the key you wish to show through.
Edit the relevant key from the left hand window by clicking the key using the right mouse button and selecting Edit from the context sensitive menu. The Planner Key property sheet is displayed. Select the Diary page.
Select the user whom you would like to associate with the planner key. You must have at least create access to this user's diary. Select the Show through onto associated diary check box and press OK.