A good place to start learning about it can be found here.
https://www.backyardchickens.com/articles/introducing-new-chickens-using-the-%E2%80%9Csee-but-don%E2%80%99t-touch%E2%80%9D-method.67839/
You need to slowly introduce new birds. Often called the "see but no touch" method.
It takes time for chickens to get used to new members of the flock. It also takes extra space and extra feeders so bullying is reduced.