Lots of planning to do. What luggage, discount motel cards, spare credit card, tire condition, spare tools/tape, emergency plans, phone numbers of contacts along the way, weather options, and a hundred other little
decisions will fill your months before. Then there is conditioning your bodies to the stess--and it will be. The need here is to spend seat time ahead to get in riding condition. Consider a Butty Buddy
if two of you are going to be on the same bike.
With a group like that, you'll either have to decide ahead how long you'll stop and where, or have someone make the decisions along the way. An option here is to rotate the responsibility to different riders so everybody spends time as the "Road Captain". Don't just travel and presume everybody will want to do something at the same time--have one person call the shots each day--important.
It would be good to also make some 400 mile trips together first, along with an overnite or two, just to make sure you have your training and riding patterns in line--you don't want to "practice" on the big trip itself.
Then there is the decision of do you just want to make so many miles per day, or are you going to stop at every "tourist site" along the way, or every good picture site. When are you going to stop to eat and what kind of reastaurants. Are you going to only sightsee after you have stopped for the night and are going to tour the surrounding area the next day. There are some other threads on this website on trips--skim through them (and don't forget my advice if you're going through Idaho!).
A lot of decisions here. Probably the most important is to have your Road Captain be fully empowered to make the decisions and everyone will agree to follow.