I'm shooting film 200 ASA.,, Three rows of family members in front of a large white lit xmas tree, with flash. Distance from first row approx.15ft. Where should the focus on the group be? What would the suggested aperture and shutter speed settings be?
Focus on an eye.
Exposure: all of them! This is a staged event and a must-have shot. Figure out the general idea using a digital camera where you can see the result right away. Then start with those settings on the good camera.
Shoot bursts of 3 on each shot, so you have 3 identical pictures with tiny differences in people's faces, to choose from.
Your exposure speed will probably be 60 or 125 to sync with the flash. Longer exposure is better if people are holding still, since you'll need less added light.
The light from the flash will then be changed via the aperture setting and by moving the flash forward and back. This is not easy to get right or to have the flash not horrible stark headlights look. You really ought to use a digital camera unless you already know what you're doing. I shot roll after roll of practice and I knew the theory about multiple flash arrangements and exposures.
I assumed you're using film because with digital you can just keep trying and immediately see the result, and the pre-defined ISO. You don't need to know the exposure ahead of time, but will figure it out with stand-ins before the people pose for you, and can immediately see brighter/dimmer.
What you need, even for digital, is how to get the lights to work at all. One flash with a group will need bounces, diffusion, or something.
So I suggest using a DSLR. Rent or borrow one that's good enough so you don't have to think about film being an option <g>.
Where should the focus on the group be?
A good rule of thumb is that you get 2/3s of your depth of field behind the focal point, and 1/3 in front of it - so pick the extreme points you want in focus (probably the middle of the front row of people and the ends of the back row of people, unless you want the Christmas tree in focus) and focus 1/3 of the way back.
What would the suggested aperture and shutter speed settings be?
You've worked out above how much depth of field you need - you can now use any of the online DoF calculators to work out what aperture you want (here's one). This isn't something we can tell you the answer to without knowing the focal length of the lens you're using (and the format size, but it's a good guess it's 36x24mm).
Shutter speed will then depend entirely on how you're trying to balance ambient light with your flash - faster shutter speed means less ambient light and the flash acting as the main light, slower shutter speed means more ambient light and the flash acting as a fill light.