How do you address all players in-game?

You would use a for loop for this.

for i,v in pairs(game.Players:GetPlayers()) do
    print(v.Name)
end

That would print the name of all the player's in game. If you notice, though, there's one special peice of code here.

for i,v in pairs(game.Players:GetPlayers()) do

What this does is says "for each player inside of game.Players, do"

So you have Player objects, not player's characters yet. Getting a player's character is easy, though. Player's have a Character property! We just set the player's character's torso's Transparency property to 1, as you would in the properties panel!

v.Character.Torso.Transparency = 1

If we put this all together we get this:

for i,v in pairs(game.Players:GetPlayers()) do
    v.Character.Torso.Transparency = 1
end

I'm not really an advanced scripter myself, but I THINK I know what you mean.

Instead of having to write game.Players.InsertNameHere.Torso.Transparency = 0.5 , you want this to apply to all players, and I'm guessing you want this to happen the second the player appears?

You would simply do the following:

game.Players.PlayerAdded:connect(function() 
game.Players.Torso.Transparency = 0.5

end)

Now of course, the script above would probably not work for your case scenario, but what happens is when a Player is added, everyone's Torso Transparency is 0.5, experiment with it a little, hopefully this helped a little.