Our two cruises have been with groups who boarded together about 1:30 -2:00 quickly and easily..... all pre-checked online. I know a lot of people try to board between 11:00 and 12:00, but from what I've heard you run the risk of long lines due to so many trying to board early and the inevitable glitches that go along with trying to turnaround 2 groups of 1200 to 3000+ passengers.
By boarding after 1:00 we were able to go directly to our cabin and drop off our carry-ons before exploring the ship. Much easier.