CO is treated the same as 4 and 5 Star Mariners and Neptune and Pinnacle Suite guests. They get priority, and are the first groups called to board. They can board thereafter anytime they choose.
As always on HAL (post and pre Covid), the target is for boarding to start around 11:30. So, to complete check in and be in the first group to actually board, you probably need to be there by 11am doing the check in process.