What i have done so far this spring, with my two batches of new pullets, is put them in a smaller enclosure inside the big chicken yard.
I use a small chicken tractor -- you could use a dog house and a puppy fence, or an old portable portacrib with out a bottom, or a piece of wire fencing tacked up across a corner -- whatever works for your situation. I make sure they have their own food and water, and I usually try to move them around a bit so they are not stuck eating the same yucky grass. After a week or two, you can either let them out into the main yard, or you can introduce the old chickens to them one by one. I had a batch of 10, and my second batch was 13, so I felt okay letting them out into the main yard all together, since there were too many to be singled out and picked on by the other hens. Your first chickens are pretty young, so if there is not a big size difference they should do okay after some introductory pecking . . .
There was another thread here in which the people introduced the chickens one by one to each other and then let them all out together -- no matter what the older / first batch will peck at the new ones, but it should be over with quickly. My second batch of pullets is settling in really well today after their two week introductory period. I usually try to let them out on a day when I can be home to watch them to make sure it is all going well. If there is one hen who is being too aggressive, you can take her out for a few days until the rest of them acclimate.
Hope that helps!