trail mix amounts for a group?

I assume you'll be providing bags or something for them to assemble the trail mix in? Multiply the amount your bag holds by the number of people then distribute that among the ingredients, then I'd probably adjust my amounts doing more of the "filler" items (cheerios, pretzels, pop-corn) an less of the other items.

My list (per person) would probably look something like this if I'm providing 8oz bags:
pop-corn- 1/2 a cup
Pretzels/peanuts/cheerios - 1/8-1/4 cup
Dried fruit - 1-2 TBS
Candy - 1 TBS

When you set up, put larger scoops/spoons with the filler items (pop-corn, pretzles) and small spoons with the more calorie dense items (candy/dried fruit)

(eta, I know that popcorn isn't spelled with a dash, but sometimes the little popcorn smiley makes stuff hard to read)