Welcome to BYC Glad you joined the flock! What you're seeing here is the controversial "pecking order", with a "head girl" showing the newbies who the boss in the flock is. Not a pleasant experience, but part of the hobby and chicken life. All you can do is keep an eye on them and make sure the new ladies get to eat and drink enough, even if it means cooping up the bully head girl for a bit sometimes during the day. Also watch out for excessive bullying, injuries etc. If that happens, remove the bully girl from the flock for a day and see if that calms her down. They should hopefully settle down within a few days and learn to live in peace with each other though.
If you're interested, there is some more info on the topic here:
Enjoy the site!