How do you find a certain player's location?

Given a Player object, we can find the position of a part of them using their Character's Torso/Head's Position.

That would look like:

if player.Character and player.Character:FindFirstChild("Torso") then
    local position = player.Character.Torso.Position;
end

If, for example, we used a Touched event, then we would have a reference to their Character instead of their player.

We would have the choice to either 1) find the torso inside of that Character model or 2) find the Player from the character model and then repeat the above code.

1) Find torso from hit part

if hit.Parent and hit.Parent:FindFirstChild("Torso") then
    local position = hit.Parent.Torso.Position;
end

2) Find player from hit

if hit.Parent and game.Players:GetPlayerFromCharacter(hit.Parent) then
    local player = game.Players:GetPlayerFromCharacter(hit.Parent);
end

In all cases, position would be computed as a Vector3 value.