I normally add them in when they are 5 to 6 weeks of age. They get to see each other in between that time. Occasionally I will get a motherly hen that will sit close to them or even bring them worms and bugs. When I see that happen I will add her in with them and allow the bonding to happen. If that happens, she wont allow anyone to touch one. But if they are without a motherly hen then wait until when they are a month or more old, and have a short roost for them. Then add them on the roost at night. The older hens will accept them quicker that way.