understand more abit will be good
Go to aperture priority mode(The A in MASP),Set your aperture to about f11(f8 if it is a bit dark),pop up your flash.you lose some details on your background but you will get everybody in focus.
If it's a stage, make the technician guy turn on ALL the lights and you should get enough light. I've done this before and it makes a huge difference. If you camera has some kind of IS or VR most people can handhold at 1/15 without motion blur and do a countdown so that the people know when to stop fidgeting.
How out of focus is it?
Bumping ISO is the best way, but if it is not enough, maybe you can sacrifice megapixels instead, by using the cropping method. One cheap solution would be to stand further away from your subject as it increases the depth of field too. After which you can crop the center of your image (area which you really want). This is useful if your camera boosts many many megapixels or you just intend to print 4R size anyway.
For group photos, make sure their faces are more or less in the same plane, because having 3 or more rows of people not only make some of their faces OOF, but create problems when using flash as some faces might be darker than others, or some too overexposed =X
If it is just a LITTLE Out Of Focus, you can always increase the clarity, or use photoshop or lightroom to do a little sharpening to make the edges and features more prominent.
In general, under low lightings, choose to shoot wide and crop if you aren't that perculiar about details and megapixels. I think that it beats having out of focused faces (:
Compact tend to take sharper images because they have generally very very short focal length which makes the depth of field so large that it is often impossible to not be able to focus on everything you want. The trade off will also be that it is difficult to isolate your subject enough from the backgrd to make it 'stand out' (: