Help with event/people photography in extreme low light situations

I am very comfortable using my flash and have shot quite a lot of events but I want to improve the photos that I shoot in extreme low light environments. For example once in a while I'll get some night time outdoor events where ambient light is minimal.

My major issue is really controlling the amount of ambient light in the background. From my point of view I am close to maxing out my 5D settings but the background still appears too dark relative to the foreground. I just want there to be a more define separation between the subject and the background. Most of my subjects here in Hong Kong are Asian and a lot of times the details of their hair will get lost in the darker background. For example here's a few photos I shot of an outdoor pool party:

Shot of the place so you can see how it is lit

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There's nothing particularly wrong with the "problem" images. They're as good as you're going to get under the circumstances.

If you want different results, you need to find a brighter background. Another potential suggestion: "drag" the shutter - use a slower shutter speed to bring up the ambient. A more extreme approach would be to have an assistant on hand with a second flash and use that as a "rim" light to separate the subject from the background. However, even a 5D can't turn night into day.

Thanks. I'm just kind of curious about the following:

1) If I shot with a 5D MKII would the results be better (since it allows me to shoot at higher ISO)?

2) I guess I'll have to experiment with dragging the shutter. How slow are we really talking? I'm thinking even though the flash 'freezes' the subjects there must be some point whereby the shutter speed is slow enough where camera shake will come into play.

Getting an assistant is not really a viable solution for me right now. At every low light event I already try very hard to move the subjects in front a nearby light source but there's always people who don't want to move.

When is blur acceptable?

Tips for Xmas Ball Please

Thanks. I might shutter drag a few of the shots but honestly it's not really my cup of tea stylistically. I think stylstically it would work better in a nightclub setting but not for most events I shoot.

Will read the other link shortly since there are so many links in that thread.

Any more ideas/tips/advice guys?

Like others said, I don't see any 'problem' with your photos.
Other than what was mentioned about about dragging the shutter, maybe you can shoot at a higher angle so the background isn't the dark sky, but the ground.

If you have an off camera hotshoe cord, or wireless poppers, you can hand hold the flash at a higher angle to expose the tops of their heads so their hair won't blend in as much either.