In my Amber game, none of the players are familiar with the setting. None. I am, somewhat, and thus far sufficiently.
So, we start with the standard amnesiac plot – you are all prisoners in this facility and know nothing. Have fun.
The players of course knew what I told them during character creation. I also told them to freely read the rulebook, and if they happen to read Amber material online, that is also completely okay. Their characters recover their memories to the extent that the players find out about things. They should still note that my Amber is, by necessity, not the official Amber, as I don’t remember that well enough. So their characters might have misconceptions and remember falsities. I get all that for free by allowing metagaming in these ways.
The organisation which imprisoned the player characters is (heavily inspired by) the SCP foundation. They have, for example, already had several encounters with [REDACTED], have sort of allied themselves with certain employee of the foundation, and attacked it more than once, generally pretty unsuccesfully due to lack of preparation and disparity in manpower. I have stated that they are free to read any stuff found there and use it to their own advantage, though my SCP foundation is not exactly what they find there, as before.
As a game master it takes some trying to not simply tell things or hint at them or whatever, especially when not playing. Games with serious secrets and hidden information are pretty much unlike the games I usually run, where there is certainly unknown information, but revealing it is not a problem. Here hidden information and not knowing are parts of the game. We also theoretically use secret notes, though actual use has been sparse, mostly due to old habit of not using them at all. I should send some dummy ones. And some proper ones, too.
As with all things, claiming that metagaming is good or bad is useless. In this case, it is mostly desired, but players are also free to stick with what their characters know. It might even be more fun that way, depending on the player. Players are free to make the call either way.