You need to make a specific effort to feed them twice a day apart from the older chickens so you know they're getting enough to eat. I recently introduced a single hen into my flock, and it took her over a month to develop the self confidence to eat from the communal feeder. I would feed her away from the others, and I doubt she would have eaten enough otherwise.
What you're seeing is very normal. The original chickens will chase new ones away from the feeder, and the newbies are afraid of the original chickens so they won't feel brave enough to eat for some time yet. But eventually, everyone gets used to the new ones, and they will begin to behave more like they belong.