How to enable Steam Family Sharing
Steam Family Sharing enables you, your friends, and your family to share games with each other while keeping your accounts separate.
With Steam’s easy-to-use feature, you, your friends, and your family can play one another’s games while earning your own achievements and saving your own game progress via the Steam Cloud.
To enable Family Sharing and authorize a shared library, you and whoever you want to share your games with first have to activate Steam Guard.