This is probably one of the toughest challenges a chicken keeper can have, trying to integrate a single chicken into a flock. You can almost guarantee they won't be accepted and welcomed.
But it's not impossible. I'm doing it right now with a two-year old rescued hen, and it's been not quite three weeks since I brought her home, and I'm stunned at the progress she's made adjusting to her new flock of twenty other chickens. She's actually starting to make friends with the two youngest members and I saw her dirt bathing with an older hen the other day. The flock mostly leaves her alone now, but she still gets chased and pecked by a couple of bullies.
It requires lots of patience, and don't give up!
Here are some things I did that might help you. At first, the new hen had her own pen where she was safe but in full view of the flock. Her food and water were inside this pen. After the first three days, I opened the pen for a few hours each day so the new hen was forced to interact with the flock. This helped her develop strategies for dealing with the flock, as well as helped her learn who to keep watch for and who she could trust not to attack her.
Regardless, I feed her twice a day inside this safe enclosure so she's assured of getting plenty of nourishment. If I expected her to eat out of the communal feeder, she wouldn't be able to eat in peace long enough to get enough to eat. This is critical. You will need to feed your little girl separately for several weeks or longer.
I also let my new hen have a few hours "rest" a day in the safe pen so she can recover regularly from all the stress she's under. During this time, I've begun to let the two youngest pullets in with her for company in hopes they might bond. It appears that may be happening.
She learned very quickly that she could count on me to rescue her from her tormentors, and I've been trying to force her to learn to fight her own battles. It's been a careful balance of knowing when to step in and referee and trying to give her the opportunity to adjust in her own way. She roosts with the others, and while it's not a simple thing, she manages on her own to find a place to roost every night, even if it's in a different place every night, according to where the others decide to roost. She's finding her place in the pecking order, and I think she's doing terrific.
Mrs K recommended you try putting a docile hen in with your girl. I think you need to try that, but do it after your girl has eaten. My girl won't eat when the two pullets are in with her as long as they want to eat first.
Hope this helps. Good luck.