Superbee you have plenty of good advice sent your way. I have the 550D so this relates specifically to your camera dials and would be the setup I would use in the situation you have described
Use the eye-piece not live view to compose & focus your shot
Set ISO to AUTO
Set Camera to M
Scroll wheel to 1/50 seconds or faster (using a 50mm lens)
Push the AV button and scroll the f/stop to your desired (for a single head shot f/2.8 for a group f/5- f/8)
Press the right magnifying button (top right corner of camera back) This will show what focus sensors are on. Scroll until you have only the middle one highlighted
Press the Q button - select evaluative metering and AI SERVO for focussing
Now your camera is set to avoid camera shake (faster than the lens) & the depth of field is set to keep focus on only those people you want in focus. The focus point is exactly in the middle of the shot. The camera will automagically adjust the ISO for each shot. You can now concentrate on composing & focusing the shot.
You will find that sometimes there is just not enough light to get a good image and this is where you give up and put the flash on.
One other thing that I find useful is to not use the shutter release button to focus. Push the menu button. Go across to custom functions. Push the set button then arrow across to 9 "Shutter/AE lock button" Select option 3 "AE/AF, no AE lock" now when you want to focus push the de-magnify button (top right corner of camera back) and take the shot with the shutter release button as normal. This is useful because you keep the focus button pushed down constantly in AI servo to track someone on the move and push the shutter release button at exactly the right time hopefully, often just a little too early or late.
With low light shots you may find you get a more consistent result by taking shots with a little extra room around the edges with the intention of cropping latter. The reason for this is that it makes you step back a little more, allowing you to use a wider aperture without having the depth of field let you down. The distance to subject is really important as well as the f/stop in giving your depth of field, it is something that didn't dawn on me as soon as it should. Hope this is of some help. Good luck