I photographed a bat mitzvah two weekends ago, and at least the weather cooperated for the beginning of the party. It was at a place with a great view to the west, and we had great sunset. The party went into the night, for two hundred people, ugh. Anyway, I have a bracket attachment and I bought a small 'softbox' that velcros onto the flash, (A Metz 54 MZ ). A small step ladder was very useful too. This is not my area of expertise at all, but that softbox really softened up the shadows. I managed to get some really nice photogaphs with it, and they were happy. I did not do a group photo of the whole party, but did do a bunch of 'table shots" which I found deeply boring, but the clients were really happy to have them.
Good luck with it, I got roped into it too, another long story, and hats off to the folks who do these things all the time. Too much stress for these once on a lifetime events for me!