I have 22 adult chickens in my coop (3 are roos) and I just added 18 chicks to them (around 8 weeks old). I just put the chicks in the coop during the day when the other chickens were out free-ranging, and shut the door. I let the adults in at feeding time, and they never really seemed to notice the flock had gotten bigger. There were a few yelps let out, but that was it. Some people add the new-comers after the other chickens have gone to roost for the night, and some add the nebies with a partition in place until all the chickens get used to each other.