Do not introduce one bird to an established flock of 7. That is a disaster, but there are ways to do this, but it will be a lot of monkeying around.
First, let all the old birds out of the coop/run set up and lock them out. Put a temporary nest near the gate, most will figure it out just fine. Lock the new bird in the coop/run. She can explore, find escapes, hide outs, and just figure out the place - let the others in late in the day, put her in a cage for protection in the coop. Rinse/repeat.
Day three- figure out who your middle birds are in the pecking order. Walk away from the flock and throw down a treat. The first birds are the top, and last birds are the bottom, and the ones in between are the middle, pick ONE of those. She goes in with the new bird back into the coop/run set up. This is the A pair. The others are still out in the yard.
Now there will be a dust up - but it is one on one, not 7 on one. And it should settle pretty quickly. Rinse and repeat. When you are pretty confident, pick a pair of the other middles to add to the A pair. Again, probably a mild dust up, but should settle quickly. If one of the B pair is very aggressive. Take her out and wait for a couple of days.
Now on a day you are going to be able to watch fairly closely, let them all mingle together out in the yard. I would expect this to go pretty easily. They have all been seeing each other for a week, they are all together at night, all though separated. ONCE in a while you will find a demon hen, that is merciless, and what is worse, she will lead others to attack. If so, pull her out and put her in the cage for several days.
If all else fails - you could put pin-less peepers on the mean birds.
Adding a single bird, is a big challenge, but it can be done, but it is a lot of monkeying around.
Mrs K