This is a list of the items I took with me on the previous trip, and it all fit either in my jacket pockets or in my 2 small side cases.

Clothes for 4-days and expected weather conditions
Jacket for 50deg to 100deg +
Tires good for 1,500 miles
Water
Power/granola bars
2 pairs of gloves
2 visors for helmet
Neck protection
Beanie
Baseball cap
Riding gear (jacket, pants, boots, etc.)
Rain gear
Riding boots
Comfortable seat
Motor oil
Paper funnels
Tire repair kit
First-aid kit
Allen key tool
Multi-purpose tool
Needle-nose pliers / wire cutters
Electrical tape
Spare motorcycle key (give to riding partner)
Emergency contact phone list (give to riding partner)
License / registration / insurance
Pen / scratch paper / change for pay phone
Cell phone charger
Mini flashlight
Immodium AD / Advil / perscription drugs
Toothbrush / razor / toiletries
Book / magazine
Maps / atlas
Camera with charged batteries
Glasses repair kit
Rags

While this looks like a lot of stuff, most of it is very small and does not take a lot of room. This all fit in about the same amount of space provided by a medium to large backpack. Also, not every rider needs every tool, motor oil, first aid kit, etc. Most of the support items can be split among the group.

For long distance trips on a bike storage space for gear can be a problem. My bike has some small hard-cases that mount to the rear, and I will be purchasing a tank-bag to complement a large bag that straps to the rear seat area. On top of that you can strap tents, duffel bags, etc. For bikes that don’t have hard-case options, there are plenty of different cargo systems out there that can be made to fit, from after-market hard cases to soft luggage. Some examples here. Since we will probably not be very far from food/lodging at any time not every bike will necessarily need to carry camping gear, tools, etc. Once the group is set we can determine who will carry what gear for the rest of the group.