Do you have off camera flash setup?
I'd actually want one more good flash unit as well to keep things evenly lit for groups. You can pick up a cheap flash gun of almost any sort for off camera for next to nothing. I have a whole box full of stuff I picked up for pennies. Can't use it on any camera I've ever owned, but it's some awesome cheap off camera flash!
Large umbrella or softbox on each, a flat ratio because it's a group and you want them evenly lit. One camera left and one camera right. If it's a HUGE group you may want a few more flashes, but I am assuming small groups of maybe 10 max.
I am not personally a fan of the reflective umbrellas because of the added contrast, add that to being dusk and that has the potential to be a lot of contrast. Then again if shot well, it could be fine, just not MY favorite way to go in this situation if you don't have the knowledge to make it turn out right.
Use a DOF calculator on your phone to make sure you are covered if you aren't all that hot at judging DOF yet. Or just stick around f/7.1 or better to be safe. If it's a huge group even higher.
I'd shoot from a tripod so you don't have to be behind the camera all of the time. That frees you up to see more-making sure no one is hiding their head behind another, etc.
I'd set up in your backyard or really anywhere that you can kind of simulate the situation and play with the light setup now so you have a good base when you get to that point.