Lets take a look at the setting from the non-flashed picture that the camera came up with (and lets ignore for a moment the crazy lighting differential from Camera left to Camera right).
1/125, F8, ISO 8000
Lets look at the couple in the middle of the scene (black guy and I think asian girl in front). This paring are basically in the middle of the two lighting extremes (bright Left & dark Right). Looking at these two they are almost correctly exposed (I say almost because the overhead light is causing chaos).
Since you are shooting a crop sensor, your DoF, by definition is wider (this is good). I don't know your FL - but let's guess 20mm and your distance to subject is 15 feet. If you plug these numbers into a DoF calculator then everything from 5.5ft to infinity is in focus - i.e. it was highly, highly, highly unnecessary to shoot at F8 and to need ISO 8000 to do so. With a wider angle and crop sensor, F4 is plenty enough DoF to get only two rows of subjects in focus.
Thus, 1/125, F8, ISO 8000 can be changed to 1/125, F4, ISO 2000 and still have a huge DoF - you could honestly at that shooting distance, and that FL also easily use F2.8 and still easily have everyone in focus - thus you could also shoot 1/125, F2.8, ISO 1000 for proper exposure.
Lets move on to the 2nd step - the lighting you are facing is brutal - i.e. the people camera left, even at proper average exposure, have their foreheads blown out from the above light - and that is why the camera, when it dumped ISO to 4 1/3rd stops below what non flash average proper exposure was it prevented the overhead lights from nuking the people on the left - and then just lit the scene with the flash - ie. better but not perfect.
Besides listening to the good comments above about moving the people away from the background, and other things mentioned, my suggestion for this scenario (assuming you can't bring in off camera flashes is the following:
1) Try and position the entire group in similar lighting - i.e. in this case, create a 3rd row of the tallest guys and move everyone into the non bright area (or good lighting if possible)
2) if after all of this, there are a few people in much brighter light, then meter on these people and choose an exposure of 1/2 stop under as your Ambient exposure to hard dial into the Camera in Manual Mode.
3) Set your flash to ETTL and take a few pictures - using FEC -1ev, 0EV, +1 EV - and pick the best one
In the exposure data I was using above, (1/125, F2.8, iso 1000), my guess is that the guys on the left (brightest part) are 1 full stop brighter - so that brings us to (1/125, F2.8, iso 500). This base exposure is 3 1/3rd stops brighter than your camera used in Automatic mode so the flash wouldn't have to work as hard - and by then taking 3 quick exposures using FEC (-1,0+1), or better yet, after the first exposure, making a better adjustment that then taking a few pictures with FEC around this better guess is the right thing to do.
So, in review:
- do a quick review, before you do this again, using a DoF calculator to give you an idea of your DoF given the FL you are likely to be shooting
- set up the group better if possible
- do a quick exposure check of the brightest lit person, snap a picture, and adjust the exposure, in Manual Mode, to peg the base exposure to not overexpose this person (pick an exposure level 2/3rds to one stop under this person)
- set FEC to -1 and take your first picture (look at it and then make a quick decision do I need more or less)
Obviously with time and an assistant, you would have two very large umbrellas with powerful flashes 12 feet in the air on either side of you and it would be easy - but that doesn't mean you can't get 80% of the way there with an on camera flash and a good base exposure.
Hockey and wedding photographer. Favourite camera / lens combos: a 1DX II with a Tamron 45 1.8 VC, an A7Rii with a Canon 24-70F2.8L II, and a 5DSR with a Tamron 85 1.8 VC. Every lens I own I strongly recommend [Canon (35Lii, 100L Macro, 24-70F2.8ii, 70-200F2.8ii, 100-400Lii), Tamron (45 1.8, 85 1.8), Sigma 24-105]. If there are better lenses out there let me know because I haven't found them.