Thick, cotton socks. For clothes, wear layers. If the sun's shining and you are working hard, you may want to shed most of them. So have a short sleeve shirt, sweater, coat. If you can just have a coat to keep of rain and wind (if you do not need more), that is good. Gives you more options.

Don't expect to have enough water to wash, other than hands and face. Bring a toothbrush, be able to keep your hair out of your face.

I assume the group is organizing the food, tents, backpacks? Getting your backpack adjusted right is the difference between a fun hike and a nasty slog. For extra food, you want to have the most calories per pound, that you can just munch. A granola mix, high in nuts and dried fruit, put in a few resealable plastic bags.

Good luck, have fun!