A map will greatly assist.
Hi, and Welcome to the Great American RoadTrip Forum.
Your best bet as a starting point is to get a large map of the USA, one which shows the routes, major urban centres and natural attractions. (I like the National Geographics map best.)
You must between the eight of you all have something in your head which you want to see. Mark these on the map first. (Little postit notes are good for this.) Once you have these on the map, it will give you an indication of a route. Then spend some time playing around in the Map Centre on the green bar above. Use the legs between the attractions you have, and see what else there is along the way. When you have some dots on the map, and given us an outline of the interest of your party, we will be able to help you fine tune the route and fill in the gaps.
One thing to keep in mind, the larger the party, the slower your progress... whether that be in one or two vehicles. And make sure you add 20% to the times in all computer mapping programs. Suffice to say, with three weeks you have a nice amount of time to see quite a bit. Just for comparison, it takes five to six days to drive coast to coast by the most direct route.
Enjoy the planning.
Lifey