How do I add a player variable to my script?

I think what you're asking is a leaderboard system. with this way you could check how long a player is on the server this session and how much gold he has.

Example leaderboard

    local function onPlayerJoin(player)
        local leaderstats = Instance.new("Folder")
        leaderstats.Name = "leaderstats"
        leaderstats.Parent = player

        local gold = Instance.new("IntValue")
        gold.Name = "Gold"
        gold.Value = 0
        gold.Parent = leaderstats

    local.time = Instance.new("IntValue")
    time.Name = "Time"
    time.Value = 0
    time.Parent = leaderstats
    end

With this you have a folder in each Player with 2 Stats called gold and time. Now you could also do an easy function to make the IntValue in Time go up 1 every second. If you do that you can check with print how long the player is on the server or make a GUI and display it there. I just gave you the beginning.

Example of Time Function

while true do
wait(1)
time.Value = time.Value + 1
end

(I dont know if this works, Roblox is down right now) But if this works you need to place it in the same script, right under my Lines up there.