I enter new chickens quite frequently. You should quarientene them. this where you seperate them and make sure they are not sick, not carrying any diseases, and treat them if they are. You should do this for about month. I know people who dont but it is better to be safe then sorry. Because you could buy a chicken that seems perfectly healthy and it could give your flock a disease. Not trying to scare you but IMO opinion it is necessary to quarientene. They after that, you can let them join the original flock. You can either put them in a cage in your pen for the others to check (i Do this a few times to make sure they get to inspect each other a little, or if you want you can just toss them in the pen. Your originall hens will mostly likely peck the new ones, you might see a few little fights. They are just trying to establish their pecking order. I would not intervene unless any of them are really hurting each other. It is usally just a few little disagreements and some pecks and then everyone is good.