I would definitely use the 50mm if you have it, otherwise stick to the 55mm end of your other lens. Don't be tempted to go wide with the large group - step back as far as you can and squeeze people in instead.
Yes - get a flash. If you're planning on buying one anyway and can get it now, that would be good. If you're going to rent, maybe rent two. You can get by with one though. You'll want to be perched slightly higher than the group, and if you can, power up the flash to +2 or +3 (on TTL - it's Nikon so I'm not as familiar with your flash lingo) and try to bounce the flash off the wall behind you (over your shoulder). The closer to the wall you are, the better.
With big groups it can be difficult getting everyone set up, so you'll probably want 3 tiers of people; back row standing, middle row seated, front row on the ground. Don't worry too much about heights except for the back row - try to have the tallest in the middle and the lesser tall to the sides. A pyramid shape for portraits is quite pleasing to the eye. Use the same pyramid principle for the smaller groups. Also, chairs are your friends. You can greatly increase the overall look of portraits by staggering the heights of people. Put an arm chair in the middle; seat someone or a couple of kidlets in the armchair, but they need to be on the edge of the seat. Then have two people sit on the arms of the chair and the rest staggered around the sides and back and on the floor in front.
Have fun and good luck!
