kirby2ig wrote:The power supply ground isn't connected to the RPi ground. How would I connect it?
I think you may be looking at things from a wrong perspective.
I'm going to assume you know nothing, not because I think you don't personally, but for the sake of anyone else coming here
So, electric current goes in circles. It starts at the positive end of the power supply, goes through the circuit, and back to the negative end (and the power supply shoves it back up to the positive end)
If there's a break so it can't get back, it won't set off.
Your meter can be part of a circuit. Put the black lead on the negative end of the power supply, be it TP2 or the metal of the USB socket or P1-06, and the red lead on the positive end of the power supply such as P1-02, and current will flow and your meter will display it.
Put the black lead on some random unconnected bit of metal, be it another PSU labeled "GND" and there's no way for the current to get back, so it won't flow.
Just because that other PSU has a pin labelled GND or earth or chassis doesn't mean it's connected to the earth in your flowerbed.
The same goes for ground on the Pi.
It's called ground because that's where Marconi connected it. He stuck a big metal plate into the ground. But ground on the Pi isn't ground. In fact the whole power supply is isolated.
So to measure a voltage on a random P1 pin you must complete the circuit with your meter, using the negative rail of the Pi