Chickens do better with friends, so adding a bird or 2 is a good idea. I prefer adding chicks vs adults to reduce the potential of fighting, your existing girl will be more willing to accept a chick and still be the "boss" You will want to set the chicks up with an isolated space in your coop area where they can be seen by and inteact with your existing girl but she can't get to them for a week or 2 as "shared cohabitation" space. Then you can turn them loose together and monitor for a day or 2. They're going to have to establish a bit of a pecking order so some small squabbles are likely. As long as she isn't attacking them in a sustained manner over time, they will adjust.