Good luck! If you can, get the flash off of the camera, if not, try tilting it up and using a bounce card (tilt the flash head to 45 degrees, with a piece of white card, say 3x5 taped to the top). Pay attention to your backgrounds; once everyone is posed, look carefully for trees, light-poles, etc that are growing out of people's heads. Once you get the composition you like, zoom out/walk back a little to open it up just so that you can be sure you've not cropped off any fingers, toes, etc. Try for a symmetrical pose if possible.... tallest on the outsides, shortest in the middle, or something like that. Check your DoF and make sure you use an aperture which is sufficient to have the whole group in focus (Every photographer should have a measuring tape and DoF tables in his/her bag). Check your ISO, WB, etc, and make sure they're correct. Take LOTS of images, move people around, try different poses, and most importantly: Ensure you discuss clothing. Aunt Sally showing up in a white dress beside uncle Bill's dark blue suit is just going to make life miserable for you. Look at the venue, and figure out what colours will work well (NOTE: I don't me which will match), so that the subjects will stand out, and that you will have a more attractive final product. Ensure you're in RAW+JPG.
Good luck!