You should be able to book for other people who aren't on the cruise. It's easiest if you have their full names (as on their passport) and birth dates, but in a pinch you can just put one name on the booking and get everything fixed once you're back home.
You typically can book more than 2 rooms, but the 3rd and subsequent rooms don't get the 10% discount and the lower deposit. They do get onboard credit, though, so it's still worth doing.
In order to keep the discount, one name of one adult must stay the same on the booking, but you can add/subtract/change every other name.