I agree with donrae. Separate the dominate hen out (if you can, put her in a cage inside the coop where they all can see each other but she can't get to them) before you add the new ones in, the new ones should be at least close to full grown before you add them. Wait until they have been accepted into the group before re adding the dominate to the flock. House the new pullets within sight of the rest of the flock for a while before adding them in. And of course provide hiding places and extra feeders & waters when you add the new pullets. Often if you have a bully, removing them for a week or two alieves some of the problem.