At the last place I was using the flash on TTL, mostly bouncing off walls or ceiling. However I didn't think about the ISO, so will try that next time and see the results.
Flash vs no flash is always going to look different, hence why some people have such strong preferences either way. I'm happy to use either, but find the results without flash much more aesthetically pleasing for conveying club atmosphere, even if flash is better for close-ups of people.
EDIY: I realize this probably sounds condescending but I don't intend that to eb the case. I'm a complete rookie myself when it comes to flash photography and ave recently even started a thread myself on it. I too originally always thought that flash photography looked artificial and preferred natural light but really when you don't have adequate natural light to work with you don't have a choice. The art of flash photography is try to mimic natural lighting, or at least make the photos balanced to the point that it looks natural. It is like HDR, most people's HDR photography is horrible, but sometimes you dal with scenes that is just not possible to capture in a single exposure. HDR done right means you will barely notice it is HDR and everything looks natural.
The difference comes down to technique, not that I know much about flashes at all. If you are good with flashes then the looks will be extremely natural.
I bet you shot at like ISO 200 and found the background dark, that is user error rather than a limit of flashes. Try to push the exposure up so you get the background balanced with the foreground, the flash is used to get a sharp image of the foreground subject with even lighting.
My worry with shooting without a flash at f/1.8 for these types of photos is the DoF will be way to thin to get the group of people properly in focus. You mentioned earlier that you had focus issues with people not being well aligned to the focal plane, even if they are perfectly aligned f/1.8 at 50mm is just not going to be sufficient. Things will look good on the LCD but when viewed in lightroom you might find much less keepers.
When people shoot fast primes wide open they tend to do it under specific scenarios, e.g. photographing the bride and you draw attention to the ring, or flowers, or eyes. There aren't other people in the photo. The exceptions are sometimes when, for example shoots, Raymond the couple with his 85mm f/1.2 wide open - he is at such a distance that the couple will mostly be in focus. Up close wide open will be very tough when photographing 2-4 people in a club.
You then have the problem of subject movement, even if you are getting 1/50s if the subject is dancing you will get a blur - a rear curtain flash will get the subject nice and sharp super imposed over a blurred subject
It seems counter intuitive to raise the ISO and use a flash at the same time but that is really a beginners mistake. You want to expose for background and use the flash to balance the subject as ell as ensurign a crisp subject even if the background get slightly soft.
