Sony posted a FAQ detailing how digital and physical game sharing will work on players' shiny new PS4's.
Upon setting up the PS4, and after the day one update, users will register their systems as their "primary" PS4. In essence, this registration tells Sony which console is the primary unit to download games by using Sony's new iOS and Android app as well as which console is the main hardware to connect with a PS Vita. The primary console also allows other players to access your digital library under their PSN ID allowing them to earn trophies for their account and access "certain benefits" of PlayStation Plus.
One really interesting function built into the PS4 infrastructure is the ability to play digital content on other PS4's. By signing into another console, players can download any number of PSN games directly to that machine, download, and play. According to the FAQ, owners can use two systems concurrently; meaning the primary and one other. In theory, if one is trusting enough, they could give their username and password to another person and vice-versa, allowing both people access to each other's digital library.
This one has been explained for us in the best way possible:
Sony also detailed a number of common questions regarding how many friends are displayed on each of its systems. PSN subscribers have gotten a bump in terms of how many friends are allowed to a massive 2,000. The PS3's shown friend allowance is still limited to 100 but Sony has addressed this by letting users know, "If the sum of players from point and your currently online Friends is more than 100, your currently online Friends are displayed by online ID in alphabetical order until the limit of 100 players is reached." They continue to state, "If the sum of players from point and your currently online Friends is less than 100, your currently offline Friends are also displayed by online ID in alphabetical order until the limit of 100 players is reached." Having 2,000 friends sounds insane but I'm sure some people are bound to fill it.
Sharing on the PlayStation 4 sounds relatively carefree in theory so here's hoping the execution is smooth and painless.