I posted this in another threat! so i am going to copy and paste. I had this situation too, and for the most part people dont mind saving a couple of hundred to bunk with someone else.
What we did was we sent out RSVP's, and for any of the singles that RSVP'd we kind of grouped together and sent out messages telling them that if they want to save a few bucks (it was 400 less to bunk up with someone) that they could room with someone else, really, how long do you spend in your room when your at your tropical paradise?? Everyone asked to be bunked with someone if there was room, we even had groups tripple up. We grouped people with similar interests and went from there, most of ours knew someone in the "singles" pool so that made it easy as well.
Now with that being said, IF they find other people by then, they can always switch around rooms when they get there! The hotel wont care who is staying where, for example we had someone book with a friend, then their boyfriend and another one of our friends booked a room, so the boyfriend and girlfriend will room together and the friends will.. even though they arent listed together in the booking... Or we had an instance where the first kid was 400 but the second one was 700, so what we did for that was we sent one child with the parents, and then listed the other child as traveling with another couple, but in the end the child will still room with the parents!
Sorry for this being so long, hope it helps!