You are outside, not in a studio. What you can use and do is limited by nature, not what you want to use.
If there is ANY wind, most light modifiers will NOT work. They would simply become sails, catch the wind, and be blown over/away. The bigger the modifier, the bigger the sail, and the more air it will catch.
Using a bowl reflector IS the same as direct flash.
I don't think you have an option for the Quantum bowl reflector size.
It's been a LONG time, but I think it only comes in one size.
Since you have no experience with this, do a trial run BEFORE your shoot.
Do TWO tests.
#1 - Coverage and exposure.
Set up your family/friends, outside, to simulate the group, and see if you get the flash coverage and exposure that you need.
Adjust the distance to the group and/or arrangement of the group so that the flash illuminates all the people.
#2 - Balanced Exposure
Then do another test at sunset, to TRY to simulate the sunset lighting conditions, and see what you need to do to get a balanced exposure. Here you are trying to balance the flash exposure AND the sunset.
Because the sun is setting, you have a limited time to do this. That is why you need to figure out #1, BEFORE this test.
You need a plan, what if the sunset is brighter or dimmer than the exposure in #1 above. How do you adjust the exposure?
gud luk