It is a lot easier to expand your flock by adding baby chicks than it is to try to integrate adult chickens into an existing flock.
I've been doing a study of the benefits to chicks raised outdoors right alongside the adult flock, and I wrote an article recently for BYC enumerating them. You can read it by clicking on the link on outdoor brooding below this post.
While adult chickens need to face the sometimes intense xenophobia of the flock, baby chicks present no threat to a flock and it's much easier to integrate them at a very young age if brooded withing sight of the flock. At the same time, the baby chicks are observing the adults and learning who's who and which adults are safe and which need to be careful around.
I begin the integration process with my chicks between age two and three weeks. Within a week, the chicks are fully integrated and there are very few problems with bullying. The secret to the ease in integration of small chicks is the panic room, a safe enclosure that affords the chicks refuge where the adults can't get into. My present chicks are two months old and still using their panic room to rest up from the pecking order, not that anyone bullies them, but they test their limits and get reprimanded by the adults.
On the other hand, introducing adult chickens is a much longer process and it can be painful for the newcomers. I wrote another article about integrating an adult hen into my flock and the steps she had to go through to be accepted by the flock. It's also linked below.
So, you can read about both options of adding to your flock from my experience and make your own choice. But I recommend the baby chick route.