I just (3 days ago) introduced 3 new 16-week pullets to my existing 7 16-week pullets. They are from the same origin and have the same hatch date. They have integrated quite nicely with my girls in a very short amount of time
You will find a lot of people will tell you that it is necessary to quarantine the new birds for 4 weeks in their own separate coop (do you have two coops? I don't) to make sure they don't have diseases that will spread to your flock. I did not do this. My pullets are all from the same place, and since they will free range together, diseases would spread right quick anyhow.
People will also recommend you keep them in a cage inside/near the existing flock's coop so that they can see each other before they are able to meet physically. I read 2-4 weeks for this stage. Again, I did not do this.
I gave my 3 new girls a bath first. I inspected them for lice and other bugs. I looked at their poop to see if appeared seemed diseased (it didn't). Then they had treats. I introduced them to my existing flock while they were free ranging with mixed reactions. I stayed close the whole time so I could witness their behavior. There was some squabbling, especially the first day, but nothing TOO violent. As long as they are not injuring each other or drawing blood, human intervention is not required; let them work it out.
I expected them to fight much more than they did, the whole thing was really quite tame. I noticed the 3 girls keeping together in a pack on the first day. The 3 new girls didn't go inside the hen house when it was time for bed (they were asleep on the ground? seems odd, I thought they would at least choose one of the roosts), so we waited until the existing flock was asleep and then we put the new girls inside the house with them. On night 2, they went in for bed time with the flock on their own.
On day 2 and 3, the new girls are no longer keeping in a separate pack and just walk among my girls freely. One of the 3 is clearly near the bottom of the pecking order, so she is a bit skittish around the Boss Chicken, but other than that it was incredibly easy and non-eventful.
This is just my anecdotal experience; I am really new to chicken farming and I can't say that my methods will work perfectly for you. Good luck!