Permission to buy songs by different users

We have multiple user accounts on our iMac, one each for a family of four. We have created an iTunes library that is shared by all. What problem we are facing is that only I can download new songs from the Store app, none of the others can. They get an error message everytime they try accessing the store app to download new songs. Why is this so? How can I allow other users to buy songs when they are logged in?
