I find it easiest if my friends or other family members make their own reservation in advance, keep it with
DCL (versus placing it with a TA at the time of booking), and then it's super easy to rebook it since the rebooking agent can transfer everything from their original reservation to a new reservation including the deposit, and you don't have to worry about carrying around their personal information and credit card.
If there is a cruise that your friends and family really want to take, I wouldn't have them wait until your next cruise since prices could rise or availability could diminish between now and when you sail. However, if they do book in advance, they would either need to pay the full 20% deposit, or only pre-book 1 or 2 people, since you can't get the deposit discount until the reservation is rebooked onboard. If there isn't a date that they have in mind, then you only need to book one person per room to secure the onboard booking benefits, and then they could shift it to whatever date they decide on.
As mentioned before, it is much more advantageous for you to rebook them versus using a Friends & Family certificate. In addition to not having the full benefits, I've heard that the F&F discount gets negated should one of you decide to change cruise dates at a future date, while there is no restriction on an onboard booking, other than you may all need to be booked on the same cruise to begin with, but either of you can change dates once booked without impacting the other.