Our experience has been good, but we did have some growing pains.. The first three additional hens, we introduced later in the day, around the time they would be heading inside for the night. The new hens spent the night under the coop. some old boards were attached around the base of the coop, and we placed the new birds on roost poles inside the coop when it was dark and closed them in for the night. At that time we kept a pan of feed and water in the coop and it wasn’t too cold or too hot, so we opened the windows and kept the coop closed most of the day. When we first opened the hatch, birds poured out like lava. Everyday after the birds all went to roost together, and we were soon able to remove the boards from the base of the coop so they had extra shelter during the day.
When our home brooded chicks were old enough we let the older ladies range, and then one by one introduced the new pullets to the closed coop. Once they were all inside and acclimated, we opened the doors and they explored the new enclosure. Guess what? no problems. Of course, chickens have a natural pecking order, but we have enough space so that it really doesn’t pose a problem. Everyone has their own opinion on introducing new birds to your flock, so I suggest you do your research and do it when you have time to monitor activities to quell any issues you might encounter. Our new birds were housed close enough to the large run that I feel that they were already accustomed to the sounds and activities going on with the older birds.