When a player clicks a TextButton, it sets off a function, but how do I target the player who clicked that GUI button?
You can use a local script to define the player as so: plr = game.players.LocalPlayer
Are you using a SurfaceGui? If so, parent the SurfaceGui to StarterGui, and make the Adornee the part you want it to be on (with a script inside the SurfaceGui). Then, using a LocalScript, detect when they click the button with MouseButton1Down (use a LocalScript because you can use Game.Players.LocalPlayer).