Reddit mentions of Impact 60" Convertible Umbrella

Did some research, here's I think my bargain basement lighting kit, and good reviews too.

• $50 Neewer flash with wireless trigger. a nice manual flash that comes with a wireless trigger. You put one trigger on the flash, and the other in your camera's flash shoe, and bam, radio triggered flash.
  
• Neewer stand/shoe/umbrella kit for $33. It comes with three umbrellas.
  
• Rechargeable batteries for your flash. I've had good luck with my Energizers, and they are cheap at $13 for a charger + 4 AA's. I'd buy two of them because the cheapest price on a 4 pack is $11, so why not spend the extra $2 for the charger to have a backup charger.
  
  There, you're good to go for wireless flash for ~$100, manual flash power, works with any camera that has a flash shoe.
  
  If you can only spend $50, then just get the flash separately for $30 and the batteries. Then you can get the radio triggers, light stand, and umbrellas later when you have the money to spare.
  
  Also, eventually, you may want to get a larger umbrella - I use an Impact 60" convertible umbrella. Big, beautiful, soft light. I have two of these; they have 4.5 stars on amazon, great umbrella. They are a little unwieldy though; 60" is a big umbrella to deal with.

The AD600 should be plenty enough. If you were doing this later in golden hour I'd say even 2 bare speedlites would be enough, but again I don't think power output is your only issue here; it's gonna be recycle times as you shoot rapidly to try and get a good frame of all 15 people. Speedlites, especially at full-power just can't compete in recycle time to strobes.

If it were me out there I'd be taking something like this and using it as a shoot-through to get a wider spread. If you have an assistant to hold your stand/umbrella that'd be ideal, but if not definitely grab a sandbag or two to weigh it down as it will certainly be windy out on the beach.