I love first timers, because we were all there once.
#1 rule: Enjoy every moment. Take your time when you walk from place to place. Enjoy the details, listen to the music and how it changes as you walk from land to land, or in the resort.
#2: Schedule downtime. Always make time to browse the shops, to actually sit and eat a cinnamon bun, to watch a show, or to go for a hot tub.
#3: With a large group, don't believe that everyone is going to want to do everything the same, at the same time. Mentally, divide the group into their needs and personalities. If you have two who are morning people, and the rest who are not, prepare to meet up with them at the chosen park later. If most are thrill seekers, but you don't care for coasters, agree to meet up at a spot later, so that you can enjoy the abundant entertainment around you while they ride.
#4: Don't believe for one second that you are all going to pull up at the gate of the Magic Kingdom and that everyone will magically know what ride or attraction to see. Make a plan, have EVERYONE take a look, and have everyone weigh in. Then come to a consensus. Use Touring Plans, Tour Guide Mike, or whomever to choose what days are best for what parks, whether you are willing to do early or late hours, and what rides to do in what order. Research before, come to an agreement, and have everyone follow the plan. It is SO easy to just go go go, and at the end of the day you've been lining up much more than necessary.
#5: Don't think that you can do everything that DW has to offer, but hope you can! You'll have to prioritize. I've never seen Tom Sawyers Island at MK, and only now, on my 5th trip, will I make it there!
#6: Stay hydrated, eat little and often.
#7: Research some more. If you think you've got a GREAT plan, pass it by the experts here and you may be surprised! I found out that my careful park planning, based on Touring Plans, excluded us from seeing Fantasmic (because, no surprise, park attendance is lower on a non-Fantasmic night). DIS'er's proved to be an extremely valuable resource in poking holes in my "awesome" plan!
#8: Have fun, take more pictures than you think you'll need, and tell us all about it when you get home!