steamsteam-family-sharing
You may authorize Family Library Sharing on up to 10 devices at a given time, and for up to 5 accounts that may then use your game library on any of your authorized computers.
From this it is unclear to me whether I can add and remove up to five friends continuously, or am only allowed to add five friends and that's it.
I would prefer some kind of source with an answer.
Thanks in advance.
You are probably applying one of the many games list filters (ie: installed games) - choose the game library you wish to view from this menu at the top of the steam window (the same way you would filter to show only installed games) and this will present you the available games from that library:
(On a train at the moment so higher quality screenshot will be added later)
Authorized Devices is a component of Steam's upcoming Family Sharing feature. It is currently only available on the beta version of Steam to users selected for the Family Sharing beta test.
When you authorize a device, then you register that device and share your Steam library with it. (Not all games can be shared in this way.) You authorize a device by logging into Steam on the device and then authorizing it through the client via the Account settings. You may be asked to re-authorize a device via email if the hardware changes.
Shared libraries on authorized devices may be accessed by any users who log into Steam on that machine. Users borrowing from a library accumulate their own playtime and achievements.
If, at any point, the shared library is being used, then any other devices sharing that library will not have access to it. A library is in use when the owner or any user on an authorized device is playing a game using a license from that library. Borrowers cannot interfere with the owner or any other borrowers who are playing the game.
On the flip side, if you decide you want to play a game in your library, then whoever is currently sharing your library (if anyone is at all) will be given a few moments and then kicked off, unless they already own the game. They will be able to purchase the game in this window of time. If you own a game you will always play it from your own library, even if the device you are playing on is authorized to borrow other libraries that have the same game.
For more details, see the official Steam announcement.
Best Answer
You are allowed to add them continously. However there is a constraint, namely that you are only allowed to add 5 friends roughly every 90 days (So when you added your 5th friend you have to wait 90 days until you are able to again. Further you can't trick the system by deleting friends before the term).
You get a message when you are in this situation which I don't want to reproduce at the moment because of the 90 days - That goes without saying :-)
Steam can't say when you have shared your games with 5 friends it's over before warning, but also it is self-evident that there has to be this constraint with the 90 days since piracy would take place in form of shared steam libraries in combination with virtual LANs (which is existing already what I strongly assume). The same issue you have on the market e.g. with time penalties on item deals.
There are several post in the steamcommunity sites, but you might've a look into this one.