It's really "two exposures". First is the FIRE, which is its own light source, and needs to be exposed so the flame looks right. If you over-expose fire, it gets very weird and pale-orange, and looks like salmon flesh. WHen the exposure is right, it looks RIGHT, like the first ought to look.
A general estimate is about f/8 at 1/125 second at ISO 400. A bonfire is actually FAIRLY BRIGHT, and can put out a decent amount of light for five to 10 feet from its perimeter, so people close to the perimeter are actually going to be somewhat illuminated.
Here is a Flickr tag collection of bonfires:
https://www.flickr.com/photos/tags/bonfire/
So...if you're a beginner, use the camera's histogram and MANUAL exposure mode to get a general exposure that looks reddish-orange on the flames; not a sickly, pale, light orange or orange-yellow. Let's say it's f/8 at 1/125 second using ISO 400.
What you can do is use the flash, also in Manual mode, at about 1/4 to 1/8 power, and stand 10 feet from the people you photograph. Focusing in the dark is tricky, and you will want to use Single-shot AF mode, and turn the Focus AF Assist to ON in the custom menus. Using Manual flash output, you can get the flash to look "the way you want it to look"...not too much flash but a little bit...
Again, the f/stop used, and the shutter speed, and the ISO level: those three things set the brightness of the bonfire.
The flash is being used in Manual mode, at a fractional power somewhere between 1/2 power, and 1/8 power, so it does not totally overpower the bonfire and bring in too much "white light".
Speaking of which: if you are shooting RAW, that's the best. If shooting JPEG, set the white balance to DAYLIGHT, so the fire looks the right color. If you have an Orange "CTO" filter gelatin, you can tape it to the flash, to make the white flash light look orange in hue.