After a beta testing period, Valve's new Steam Families is now available to all users. It allows households of up to five people or family members to share their game libraries. The new system includes the ability to pool multiple copies of games together so more than one person can play a title if there's more than one copy in the greater Steam Families library.

Expanded parental controls, including the ability to allocate 'Child' and 'Adult' roles, are also available. Adults can limit what games can be accessed, set playtime limits, and restrict access to Steam features like the Store, Community, and Friends Chat. More in-depth and robust parental controls are a big part of the new Steam Families system.
Another significant parental feature of Steam Families is a dedicated purchase request system for Child accounts. This system streamlines the process of approving and buying games for kids without using Steam's current 'gifting' system at checkout.

How does this apply to friends, especially those who live in the same house or a group of friends who live in the same city? Valve is keeping track of Steam Families and has clarified that the system is intended for immediate family members. It has also put checks in place to ensure that the system isn't abused or used by people to expand their Steam library sixfold.
"Steam Families are intended to contain your immediate family. As major life events can change who lives in your household, it is understandable that some day you may need to join a new Steam Family. Adults can leave a family at any time, however, they will need to wait 1 year from when they joined the previous family to create or join a new family.
Children in a Steam Family cannot leave the family themselves and must be removed by an adult in the family or by Steam Support. As it is rare that a family member leaves the family, each Steam Family slot has a cooldown of one year before a new member can occupy that slot."
A cooldown system is very PC gaming, but it's a shame that there doesn't look to be an easy way for friends to get the most out of Steam Families.