This is a tough situation. You need to photograph an entire team in the shade, with a full sunlight background. First, you need to expose the background correctly. Let's say it's ISO100, 1/125, f16. Ideally the background should be a little underexposed. so let's say you want ISO100 1/125 f22.
Now you add flash. The shutter speed could be bumped up to 1/250 for most cameras, so this means you need to do 1/250 f16 with your flash. Quite a bit of power, especially if you need to light the entire group with a large umbrella.
Let's say that you use a Canon 580EX zoomed at 50mm. This gives you a GN of 50. To calculate the aperture at, say, 5 meters, without any diffusion, at full power, you'll need 50/5 = f10 - this is insufficient to get the exposure you want. So you add another flash and double the illumination to get you close to f/16. Now you have to account for the umbrella, which will eat up 1 fstop at least. You can't do it with two flashes and underexpose the background. Perhaps high speed sync will get you there, but power is greatly reduced, so I am not sure two flashes will do it - worth trying.
I have used a high power strobe, 1200 ws, and this will give me the ability to overpower the sun with a modifier without breaking a sweat. So that's a possibility if you can borrow one.
Here is an
image I did in full sun, f/22 with a powerful studio strobe, underexposing the sun by 1 f-stop. In order to get shallower depth of field I would have needed a ND density, say three fstops, and dial the exposure down to f8.