Event handling over a peer-to-peer remoting system

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I'm developing a multi-player game in C#. Player A hosts a "game room" and creates a game object. Then players B and C gain remote access to that object over a TCP connection. (Assume everyone knows everyone else's IP address). My issue is that player C's UI needs to be updated when player B calls a function on the remote game object. When the program tries to register player C's handler onto the game object's event, I get an exception telling me that my security settings are too low for deserialization.

(Specifically: Type System.DelegateSerializationHolder and the types derived from it (such as System.DelegateSerializationHolder) are not permitted to be deserialized at this security level.)

The message suggests using security certificates to get around all this, but I don't know how this is done. Can someone instruct me on how to get this working? I would really rather not have the host player have to gain remote access to each other play so it can call some "updateUI" function on each player.