I'm a newbie, haven't even gone on our first cruise yet, but this is what happened to us.
My brother booked his cruise about a week before us, so about 8 weeks before we sail and got the GTY rate, he also got a cabin # at the same time.
A week later he talked us into going with them, I was able to book one of the last cabins on the boat, we are sailing March 30th on the Disney Wonder! Anyways we didn't get a cabin # right away, I called about something different and during that call I was able to link our to reservations together so we would be sitting together at dinner. Anyways the next day I got our cabin # and we are two rooms away from my brother! So we are pretty happy with that!