If there are discrete light sources in the frame, like small light bulbs and
such, then remove your UV filter if you use one and use a lens hood instead
unless the environment is particularly dangerous to your lens.  UV filters
are often the cause of strange spots in the image under these circumstances.

If the room is not too big, you might try pointing the flash straight up.
Often this gives a nice result where the room appears well illuminated in
the image.

Keep your shutter speeds high enough since you don't have an image
stabilizer or a monopod or tripod and also because the people will be in
motion.  Don't be afraid to use the higher ISOs.  You can apply a bit of
noise reduction in post processing if it is too bad.

If you shoot RAW, it will be easier to correct white balance issues if
needed.  Sometimes you want to capture the colour of the light and use the
colour casts, sometimes you want the colours accurate.  If it is the latter,
then take a picture of a piece of and 18% grey card or even a white piece of
paper under the same light at some point so you can use it as a reference
for post processing.

Remember that the guide number of the flash is specified at ISO 100. At 200,
you get 1.414 times as much, at ISO 400 you get double etc.  You can fill a
quite large room with a full power blast from your flash at ISO 1600 and a
large aperture.

This is not much of an answer but I have a 28-105 3.5-4.5 This lens
seems to need a 5.6 or higher/smaller aperture to be sharp. At least
mine does. Don't know about the Sigma but even if it takes an F4 to be
sharp you are ahead of the game with it. I would shoot RAW if possible
because of the added flexibility. Set the shutter speed manually and
keep it 100 or above with the Sigma and 160 or above with the Cannon to
help with camera shake.

Try a fast prime lens.  A 50mm f/1.8 costs way less than your
flash did.  Or lend one.  The zooms will probably not even
focus in so low light.

Outdoors you may try to bounce the flash of walls, but it'll not
work always (distance, etc.) and will not always give you the
right results.

Since you shoot flash, use the M mode.  If the subjects are notably
darker (2, 3 or more stops) than the background, expose for the
background (maybe one stop underexposed) focus on the subjects and
light them with flash.  Sitting on the ground I have gotten away
with 2 secs exposure, since the background was well out of focus.

You have an LCD, use it.  Check it often.  See if you get what
you want.

Remember that the focus point is important to the TTL flash
metering, so be prepared to focus, then move the focus point,
if needed; focus and recompose may not work well for you.  Read
   http://photonotes.org/articles/eos-flash/
and understand the salient points.  And try them out beforehand.

See if you can get away with a monopod.  Lean firmly on walls.
Rest your camera on an umbrella.  Use a bean bag, press your
camera and the bag against a tree, for example.

Remember the light fall off.  Do not try to shoot deep formations
(unless you can bounce the flash of some wall or roof or
what-you-have (even glass roofs can work!)), position yourself
to flat formations.  e.g. if there's an oval table, shoot from
above and one side, not from one end.

Use available light as much as possible, flash should mostly
fill in, if at all possible.  Be not afraid to use ISO 1600, if
that's what it takes.  Especially with RAW you should be able to
post-process.  (and filter out some noise, but don't overdo it.
Grainyness is OK.)  Pure flash gives a very dark background.
Maybe turn the flash down a bit, or a bit more.  Watch your LCD,
see if you like it.

Be aware that contrasts at nights can be very strong: you can
_very_ easy blow out highlights.  Be sure you want that or handle
them. (You can however turn the image to B/W, where that does
not lead to color shifts.)

If there is artificial light, you may want a colour filter for
your flash unit, otherwise you may have ... fun with a bluish
(flash) and greenish (neon) or reddish (incandescent) light source.
OTOH, without close artificial light, the outside is very blue
(the atmosphere, you understand ...)  Candlelit, warm faces versus
the blue background can be interesting.

Watch your exposure time.  Moving people can be a problem.
People sitting (or standing, even), listening are easier.

-Wolfgang

Thanks for all of your posts!!!  Event and photos turned out great for an
amateur like me!