I currently have 5 that are about 2 months old and I am preparing to add several that are one month old and a couple that are 3 months old to the original flock of 2-month-olds.
Here is what I was told; hopefully keep everyone new and old where they can see each other, but not have access to each other for at least a few days then add them together at night when they are tired and cuddly and less likely to fight. Keep a close eye on them over the following few days. Some fighting and establishing of hierarchy is normal, but if it gets excessive, you may have to separate and try again in a few days, but in the meantime, try to keep them where they can see each other.
Now, bear in mind I haven't actually done this yet, so if anyone has any advice to share, I would be interested to hear it too.