I would see how far you can go with ambient light first, then add some flash as fill

• Aperture - set to f/1.8
• Shutter - set as slow as you can hand hold, 1/15 or 1/30 perhaps
• ISO - set to get approximately the correct exposure, may have to go to 800 or higher.

This may get you there. If so you can possibly use slightly lower ISO or faster shutter.

Then I'd start to think about flash, especially if ISO 800 and 1/15th aren't quite enough (or subjects are moving and you can't go that slow). If you have done the above and ambient light is almost enough, then you just need some fill flash.

• bounce of ceiling won't work if ceilings are high, but a good option otherwise
• if you have high ceilings you can try walls
• failing that, direct flash, dial down flash compensation (try -0.7 for a start) to avoid light being too harsh.

Flash will fill in some shadows and help freeze the action that might otherwise blur from your slow shutter speed. Once you've added in flash, you can think about moving ISO back down to 200-400, or if subjects are moving, you might want to move shutter speed up to 1/60 or faster.

Edit: having seen your sample image, it's not that underexposed. Judging by the people on the sides and background, who wouldn't be lit much by the flash, there is enough ambient light around. Just dial down the flash a bit if it's too harsh.

I'd suggest you get them to move together a bit, and you can crouch down. Those two things will eliminate all those heads in the background.