dunnow where you are, but ill assume the sum is going to be setting.. be aware of it, and dont trust the meter to be able to sort it out either... set your flash to ON.. not auto... its the lightning symbol.
this will help reduce shadows ... although if your right in their faces, itll flatten them.. the faces will be slighly overexposed, the wrinkles will dissapear and the girls willove you for it... but its not good...
try to keep out of the subjects eyeline until you want their attention. half press the shutter, and lock the exposure, then, grab their attention, and as soon as the turn and smile, snap... its preloaded, so no shutter lag as the camera hunts for focus.
i prefer to stand back, and shoot with a medium zoom, between 90 and 135 mm...it allows for goo dsepration between subject and background...
and keep watching... anyone can take a photo... its easy. buy a D90 and press a button... but thats only a start... you need to put your victims at ease, make them smile.. tell them it wont hurt, itll be all over in a flash... pardon teh pun... but you have to develop people skills... introduce yourself... hi, im taking photographs for BLAH... can i take you picture...would you mind?
people rarely mind if you ask...they smile and co operate... and they remember you... youre than funny bloke with the camera... and towards the end of the night, when everyones drunk...and the smootchy records are playing... you walk teh floor and shoot the couples... done properly it generates work like a snowstorm...
oh, take spare batteries, and as much menory as you have... and shoot in the highest resolution you can, RAW if you have it... then run every image thru an editor, i sugget you try Fastone Image Viewer... its simple easy ti use, and Free...
good luck...