How to take portraits in this background with dslr?

I have nikon d3200. I turned on flash but it didn't really make a difference and people i'm trying to take a picture of, they come up all black. I'm on AUTO mode.

Fill-in flash (or long exposure flash). Measure exposure for the background, then trigger the flash to light up the people in the foreground.

Get yourself off stupid more (auto) and on one of the manual or semi-manual modes. For this type of shot, I find manual mode to be better.

Set your metering mode to point metering
Meter the background,
Set your shot to properly expose the background,
Focus on your subject (make sure your flash is turned on)
Shoot your subject.

The problem here is that you'll be using the pop-up flash and that will create harsh shadows and glare but there's not much you can do about that… Try getting a diffuser for your pop-up flash but they only help a little.

Flash reflects on the glass; set manual mode, ev positive compensation