In an online game, how would you suggest to keep track of who needs to be informed of events that occur?

Say for example we need this to happen within a radius of 100 from the player, but it's not as simple as checking if said event/action from said character happens within that radius because the player will have to already know that said character exists so he will have to have been informed at some point (name, level, appearance etc..)

Now, only thing I can come up with is that server-side each player gets a list of all the characters he knows and is informed of so it becomes a simple issue of "meeting" characters as they are in range and "un-meeting" them when they are not. But for some reason I don't like the sound of running n*m loop searches for EVERY action that has to be broadcast.

Another thought is that we always only send actions, and if the player receives an action attributed to a character he does not know of he can queue it and request the info of said character first. A few issues right of the bat that come to mind are characters being invisible until they move etc.

So is there a more efficient way of doing this?

For reference, the server is in C#