Keep your new hens away from your current hens for at least three weeks to be sure the new hens don't have anything that can pass to your current hens (incubation period).
From there, see if there is a way you can seperate the two groups but keep them close enough to see each other. What I do is put the new group in a chicken tractor in the run of the other group. That way, they can get used to each other without being able to fight.
It is very stressful to them when you add new members because they have to hash out a new pecking order. Introducing them with a barrier before you let them all together is the least stressful way.