Hi everyone! I have some answers:
Regarding the Early Booking Discount: As the ship fills, the pricing will increase. There is really no way to know how long these prices will be valid, but it is best to book as far in advance as possible to get the best pricing. They do not put a specific timeline on getting the Early Booking discount.
Regarding booking for just one person now, to get the lower deposit, and adding people later: You can do this but keep in mind that everyone added later will be added at the current prices on the date they are added. Additionally, we will need to be concerned about capacity limits on the ship. This does not just mean the number of berths in the room but also relates to space in the lifeboat station, dining room and the kids clubs. Once these limits are reached, you would either need to switch cabins or switch categories, or even run the chance of not being able to add more people. But, if you add the extra people early enough, this can be done.
Regarding booking a "Guarantee" stateroom, in hopes of getting an upgrade later on: No, Disney does not offer this upfront. If there are rooms that can be assigned, they will be assigned. You may get a guarantee room assignment but cannot request this if there are rooms available in a particular category.
Jen: DCL has offered Group Rates in the past, but they are very minimal discounts. Currently they are not even offering them for 2011. But, the Early Booking Discount is much better than group rates have been in the past anyway.
Let me know if you have any other questions!!