There are literally thousands of locations for people photography in Edinburgh. Here are a couple of my suggestions though.
A walk down the Royal Mile is great, lots of different busy people in nice surroundings.
Princess street is always busy with the castle high in the background.
I have taken a couple of people on Calton Hill too, people seem in a trance looking at the view.
If you need any other suggestions, directions etc. drop me a PM i'm happy to help anyone visiting Edinburgh.