Large Group help.

I am attending my Grandparent's 60th wedding anniv/80th birthday party this weekend, and need some help.

I'm sure I will be asked to take a photo of them, their 6 children, 20 grandchildren, & 11 Great-Grandchildren. That's 39 people in one picture!

I have a 300D(hacked), a 420EX, and not sure what lens to use.

Here are the lenses I own:
18-55 Kit Lens
50mm 1.8
80-200 II 4.5-5.6
Tamron 28-75 2.8

The party will be in a dimly lit reception hall. There is a stage, with no stage lighting, that I can utilize to position everyone.

Will I be better off renting a different Flash, or Lens, or can I get away with one of my current lenses?

The rest of the evening, I will be roaming around with my 50mm and/or the 28-75.

Any suggestions, advice?

Thank you.

The 28-75 f/2.8. Put them on the floor while you stand on the stage - looking down will make it easier to see all the faces. With kids, that'll be easier than if they were all adults too. You'll probably want a tripod too. Assumption - 3 rows of people; first one sitting, next two standing, with even numbers of people, for a 26 foot wide line. You'll be able to get them closer if you have 4 lines, first sitting on the floor, next in chairs, and two (shorter and taller) standing.
I think you'll need a bigger flash than the 420 also to go 3 rows. My back-of-envelope calculations say you'll probably be 35 or so feet away from them at 28 mm, and at 28 mm zoom and f/2.8 the 420 maxes out at 30 ft. A GN of 32 (meters) at 28 mm is about what you'd need for that; a 550, using manual zoom on the flash to 50 mm (allowing for IT), will be comfortable (GN 42 meters). Even at the 35 mm zoom setting it'd work (GN 36 meters), I think. I don't know if the Sigma 500 Super allows manual zoom settings. If it does, it might be possible (but marginal). But the 420 doesn't have manual zoom, so that wouldn't be an option (@28 mm, GN 25). It would play nicely for candids, though.
If you go to 4 rows, that would get you down to about 25 ft., just within the 420's range. A benefit will be that the faces will be larger.

Another suggestion ... if the weather is fine, ask them to step outside and take the group picture there.

Then you might also try to use the 18-55 at f/8 (if you have a sharp copy).

Best regards,
Andy

You could always look into buying some cheep halogen lights from hardware store to add light.
They are hot and just plug into available outlets but can add alot of light.

Great information Jon! Thank you.

Steven, never thought of that. I have a few in my basement "studio". Thanks.

Andy, If it wasn't March in Northern New Jersey, I might try. But wrangling 40 people into the cold might be tough. Thanks though.

rent a few studio lights and bounce them off the ceiling (if it's white)