If you weren't doing this already, use slow sync flash. Then you can use the flash to illuminate the people in the foreground, while at the same time exposing correctly for the cityscape in the background. You will need a tripod (or equivalent way of keeping the camera steady), and this way you also have the luxury of choosing a low ISO.

As for the awkward shadows under their nose and eyebrows, this is just generally the result of having direct (non-bounced) flash.

If you only have the camera's built-in flash there's not much you can do about this except to try and move further away and crop/zoom instead. But since you have a hot-shoe flash, assuming you can rotate it, you may be able to swivel it and bounce it off a white clipboard or something. When you bounce flash, it doesn't always have to be bounced off a wall, you can always bounce it off a smaller object. Even bouncing it off a white business card will give it a slightly softer quality than direct flash (probably better than a diffuser you may pay good money for). Try, for example, bouncing it off the pages of a book or newspaper - as long as it's mostly black and white it should be good. Try bouncing it to the side instead of above for a more flattering angle to the shadows.

It sounds like you're probably not interested in more elaborate flash set-ups such as multiple flashes or off-camera flashes since this was just a casual outing with friends and you need to carry the stuff with you, but if you are, there's a whole other world of stuff you could get into!