Open the Steam menu in the upper-left corner and go to Settings.
Choose Family from the left-hand navigation pane.
Under Family Library Sharing, check the box next to Authorize Library on this computer.
When you log out of your Steam account, anyone using that computer will be able to play games from your library.
If you want to stop sharing access with a computer:
- Log into your Steam account from the machine you want to remove.
- Access the Family Library Sharing settings from steps 1 - 3.
- Uncheck the box to share your Steam library.