How to make a particle emitter for all players?

Well first it would be a good idea to make sure the Player and it's character have loaded in. Then you'd be able to implement a particle emitter into the torso

Here is a fast and efficient way of completing the statement above:

game.Players.PlayerAdded:connect(function(player) 
    player.CharacterAdded:connect(function(char) 
        if char then
            local torso = char:FindFirstChild'Torso'
            local speed2 = Instance.new("ParticleEmitter", torso)
        end
    end)
end)

Note: This should be put in a Script, not a Localscript