Simply follow the steps on this tutorial...
- Click the SHARE tab at the top right corner
- After clicking SHARE, you will get a pop-up. Click the down arrow and choose CAN VIEW, then type in the email addresses of those people you allow to view
- Click the tab DONE after you have type in the email addresses, and now if someone not invited try to open the file to view, he or she will see a YOU NEED PERMISSION message: