Before you share your account, you must enable the “Steam Guard” feature, which requires you to input a code — received by email or text message — when someone logs into your Steam account from a new computer.
To activate Steam Guard, log into your account and open Preferences.
In the Account tab, click the button labeled Manage Steam Guard Account Security…
Next, select whether you want to receive your Steam Guard notifications via text or email.
With Steam Guard activated, you will be able to share your library with another computer. To do so, you must log into your Steam account on that computer and go to your Preferences. Afterward, select the tab labeled Family.
Check the box labeled Authorize Library Sharing on this computer.
You should see a list of accounts that have also logged into that computer in the past. Check whichever ones you want to share with, then click the box labeled OK. Now, when your friend logs into the selected account on that computer, they will be able to download and play games from your library.
Keep in mind, however, that there are limits to Family Sharing: Two users cannot access the same library at the same time. In the event that you and your friend both want to play a game from the same account at the same time, the library’s owner gets priority.
