Help with a Function?

To get all the players you can use :GetChildren() and pairs to get all the children. This is an example of GetChildren.

local workspacechildren = workspace:GetChildren()

print(#workspacechildren) --Should print how much things are direct children of workspace.

This is pairs:

for _,v in pairs(workspace:GetChildren()) do
    print(v.Name) --Should print all of the Items in Workspace.
end

Now we get all the players:

for _,v in pairs(workspace:GetChildren()) do
    if v.ClassName == "Player" then --Just incase.
        print(v.Name)--Print all of the players names.
        v.Character.Torso.CFrame = CFrame.new(0,0,0) --Teleport the Characters to 0,0,0
        Instance.new("ForceField", v.Character)--Give the players forcefields.
    end
end

Just edit the above script.

I hope this helps!

EDIT

Perci1 Found an error. I meant to say game.Players:GetChildren().

for _,v in pairs(game.Players:GetChildren()) do --I just copy and pasted my last script so I didn't have to type the whole thing again. I should be more careful next time! BTW I do know there are no players in workspace. I also know that ":IsA("Player")" works also. It's just I'm used to "ClassName" and I used "v" instead of "player" like how adark did because it's just shorter.
    if v.ClassName == "Player" then --Just incase.
        print(v.Name)--Print all of the players names.
        v.Character.Torso.CFrame = CFrame.new(0,0,0) --Teleport the Characters to 0,0,0
        Instance.new("ForceField", v.Character)--Give the players forcefields.
    end
end

Lord's answer is incorrect. Like Perci commented, there are no Player objects in the Workspace.

To get a Table of all the Players in a game, call GetPlayers on Game.Players. This functions identically to GetChildren, except that it will only get Player objects:

for _, player in ipairs(Game.Players:GetPlayers()) do
    print(player.Name)
end