It sounds from your OP that you will shoot couples photos in front of a backdrop with lights, and then candid shots roaming the floor. No giant group shot. Is that right?
For roaming shots, in a large party setting, I use on-camera flash on a bracket, and one or more off camera flashes held by assistants. These are placed behind and to one (or both) side(s) of the small group I want to shoot. They are pointed at the ceiling, if it's low enough, or pointed toward the back of the group. This provides rim light (separation light), and lights the area behind the subject. Looks great. The on camera flash is on TTL, set to Master, and the off camera flashes are on Manual, set to Slave, and the camera is set for correct exposure using the manual flashes. The on camera flash TTL provides the proper exposure on the subject. (Watch those dark suits -- I often shoot with a -2/3 setting.)
If I can't have assistants roaming, or if there is a podium for speakers, I set up one or two (or more) lights along one wall, on tall light stands. These are also direct lights, no umbrella or box. Set them to manual output, and use them as either back lighting, side lighting, or main lights, depending on your position relative to the subject. I use an on-camera flash set to TTL -1 for fill, and fire the remotes using Pocket Wizards.
In many cases I will also drag the shutter a bit, just to bring up any ambient fixtures in the background. Yes, they are often too warm, but that's what people expect -- and they look better than if I shot at 1/250 at f/8 and the entire back of the room went dark. So my exposures are usually in the 1/30 at f/4 range, around ISO 400. Depends on the room. If the event happens during the day, life gets easier, but most of our big events are at night.
A note on umbrellas and soft boxes: for large groups and big rooms, direct flash actually looks pretty good. You just have to make sure it's filled properly, either by other direct flash or by on-camera flash. Think about good basketball lighting -- that's four direct lights, cross-lit (one on each corner) so everything has main light, fill, and rim light, no matter where the player stands. That's the goal in party/reception shooting.
Good luck. Hope you are charging them lots of cash.