We added 2 new girls to our existing speckeldy when her partner died.
We were advised to put the new birds directly into the roost after dusk and shut them in together. Apparently they never fight in the roost and will have plenty of time to get to know each other in the dark! Leave them shut up with no food or water until after lunch. Then let them out - they will be so hungry and thirsty they will eat/drink before doing anything else, and by the time they are satisfied they will have accepted each other.
I was a bit worried about leaving them shut up, but did as we were advised by the breeder we got both sets of birds from. I was also worried about our existing girl being picked on. But it worked perfectly and they were fine. There was no pecking or bullying. At first, the old bird was boss, but after a while the biggest, one of the newcomers, took over top spot.
I know we only had 1 existing chicken, but the breeder said this is what she advises when adding new birds to a new flock in general.
Good luck!