Large Group Portrait

The photo you included in your OP is just ambient light??? If so you might not need much flash at all.

I'd bring a ladder and shoot down on the first few rows. Lights in front as high as possible so the shoot down on the first few rows and gives you more even lighting for the bottom half of your picture. The top looks light you could possibly us the bright walls to bounce and broaden the second light. Soft boxes will reduce shadows but not do anything for the 1 over r squared light falloff.
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Ken Eis
http://keneis.zenfolio.com