Say there is a arena server with 4 players, player 1 is facing 2, 3 if facing 4, is it possible to only draw player 1 to player 2 and player 3 to player 4 and vice versa?
That is confusing, do you mean when one is in the view or literally when they place their crosshair on them?
If you want when one is in the view I suggest you using this: http://wiki.garrysmod.com/page/Entity/Visible
If you want when one is directly looking at the entity I suggest using this:
Then you can override the entity drawing using: https://wiki.garrysmod.com/page/Entity/SetNoDraw on the client side