Sharing iTunes purchases
Copy the protected music file as you would normally.  When you play for the first time any protected files from a specific iTunes account, it will prompt you to authorize the machine.  I believe the current limit is 5 machines, and you'll have to be connected to the internet when you enter your credentials, but after that it will be able to play any song from that account.

You can also look into iTunes Match, though honestly I've encountered so many issues, I can't recommend it outright.