When introducing new chickens to your flock there are a few things you can do to ease a very stressful situation for both the new, and your existing birds.
If you can it is best to get your new chickens in pairs or groups, place the newcomers to the group behind a fence for a few days to enable them to get used to each other and their new surroundings before being introduced to the main group.
Introduce them to their new home at night. Chickens won't fight at night so placing them amongst each other after it gets dark gives them a chance to get used to the new smells without conflict.
We have all heard the phrase "pecking order" well chickens have to establish an hierarchical order and introducing new birds means that they all have to establish their new order amongst themselves. For the most part you just need to let them get on with it but if blood is drawn remove the injured chicken from the flock. Chickens are drawn to blood so it will cause the other birds to pick on and bully the injured bird if left there.
If you have bird that is particularly aggressive and bullying other birds you can try removing it from the flock for a few days before putting the trouble maker back in.