Hi sp my DD1 started at indie in year 7, she also didn't know anyone else at the school and again a lot of her fellow classmates came up from juniors so all knew each other already.
Tell your DD not to worry as to be honest a new girl in the class is a novelty and so all the pupils will be especially nice and friendly whilst they all form new friendships and groups. My DD found that friendships that had been formed in juniors changed in seniors as the girls themselves changed and made new"best friends" so my DD was easily able to fit into the new class hierarchy.
She also kept in touch with friends from her primary school who were her "BBFL"
but gradually over time they all (her included)naturally moved on and she now mostly just communicates through msn whilst seeing just 1 friend from primary. Had I told her at the end of year 6 that this is what would have happened she would have vehemently denied it and said she would be close friends with them for ever however they change so much when going to senior school and my DD is now extremely happy with a completely new set of good friends so tell your DD not to worry and look forward to the new friendships she will make.