If your neighbours will tolerate the noise, introducing a rooster to your hens can be worthwhile. Roosters look after their hens and create a more natural social group. You also need a rooster if you plan to breed chickens. Introducing a rooster is usually more straightforward than introducing a hen, as a single rooster normally won't have to establish his place in the hierarchy. You should still take your time preparing both the hens and the rooster, as you will be interrupting an established social group.
Construct a small temporary pen with a coop for the rooster and place it within the main pen. Alternatively, purchase a small pre-made chicken run. It will also be useful in the future if you need to separate a chicken from the flock for any reason.
Place the rooster in the temporary pen. He can see the chickens and they will become accustomed to him, but they have no way of fighting. It is fairly unlikely that hens will fight with a single rooster, but it is possible, especially with a hen that has been top chicken for a long time.
Provide the rooster and the hens with extra grain or other treats so they associate each other with a pleasant experience.
Release the rooster into the main pen after two or three days. Provide a food treat again. Observe the birds for about ten minutes to ensure there are no fights and check on them once an hour for the rest of the day. In the unlikely event that there are squabbles, put the rooster back in the small pen and give the chickens a week or longer to become accustomed to each other.
Don't introduce a grown rooster to young hens -- wait until they reach laying age.