Any travel agent can tell you what ships they have group space on and you can go from there. There is no need to actually be part of the group, just get the benefits of the group booking, I've done them myself with no problems.
Donna's right. Sometimes my agent asks me to wait a couple of weeks since the group bookings are in the process of being booked, but she's very good about checking with the AmEx cruise desk in New York. She also checks the specials deals which (for some strange reason) are anounced, on RCI and Celebrity anway, on Tuesday. Anyway, whatever she does, I'm completely in the loop.