Like many other species of animals that roam this earth, chickens have a certain order amongst them that determines who is in charge, who is accepted, and who is not accepted. This 'pecking order' is what governs behavior of one chicken towards another or a several others. Because of this, introducing new chickens to your flock requires diligence and can be a little tricky at times.
When it comes time to introduce chicks to your flock, be prepared for unfavorable reactions from established hens and even your rooster. This can include harsh pecking and even killing of chicks. Because this possibility exists, putting off introductions until your chicks have grown to a size comparable to that of the rest of your flock is best for their well-being.
If you are introducing a new hen, it is best to keep her separated initially until her presence has been accepted. Keep her in a box (such as a dog crate) near but inaccessible to your other hens. Let them get used to her smell and presence before letting them getting acquainted beak to beak. Take note of the conditions around her enclosure before you let her out; if it is clear that there have been some attempted scuffles or posturing, wait to let her out until you see that has stopped. Signs of this behavior are a lot of loose feathers or other hens walking near her with feathers puffed up and making attempts to peck her. You should also be able to tell if your new hen is stressed; if so give her more time to get used to her new surroundings before letting her out. Your rooster will probably accept her gladly, but you have to prepare for the established hens to object to sharing their rooster.
Should something happen and you lose your rooster to death, replacing him is less complicated than adding a new hen. Adding a new rooster in the absence of a deceased one usually goes over fairly well and hens will continue laying eggs. You need to be certain, however, that your new rooster came from a place where he was properly socialized. An aggressive rooster that is a danger to your hens is the last thing you need and should be avoided.
Something else to remember is that chickens are social creatures and need greatly for the company of others of their kind. When a flock mate dies, his or her loss can reverberate throughout the group. Surviving chickens may stop eating, pull at their feathers, or even die of depression after a comrade has fallen. Being mindful of the complex bonds between chickens is the best way to tend to their needs for socialization and keep everyone happy and healthy in the process. Also keep in mind that, just like people, certain chickens will never get along no matter how much you encourage them, so don't get frustrated if you experience two hens who do not gel with one another. The best you can hope for is a peaceful coexistence for all.