If you are the event organizer, you need quite a few power outlet strips and maybe a carton of three into one outlet adapters. Or you need to tell everybody that you recommend they bring enough power capacity in powerbanks to provide for themselves.

This might be the best price if you are buying dozens of them. I bought some of those a year or two ago, they are as good as any of the others.
https://www.ikea.com/us/en/p/koppla-...hite-70086401/

The last time I did a week long van supported trip (participant, not organizer), I think I had one small powerbank to keep my phone charged. My GPS uses AA batteries, brought enough for a week. My taillight and my headlamp (for my head in the campsite) did not need replacement batteries.

But I do two week long kayak trips, bring enough AA batteries for two weeks on those trips. That is in part why I use a GPS that takes AA batteries instead of an internal battery.