you might also try having a playdate at your house, on your LO's turf and see if that helps.
Ask DD if there's a girl or boy in her class she likes/wants to play with and see if you can arrange a playdate. If she can't identify someone in the class she likes, ask the teacher for someone she either plays with or is a class leader.
I agree with this completely. When DD#1 started preschool, she was terrified- the boys were rowdy and she was afraid of them hurting her. I started having playdates here and she was confident and she was able to find common interests and then had the confidence to carry over at school. Additionally, DS was significantly language delayed and even though he is nearly caught up and chatters at home, he only parallel plays at school and doesn't talk. Yesterday at DD#1's tae kwon do class, there was another boy his age watching a sibling and the two became best buds, playing with a toy DS brought, turn taking, and in 20 minutes of playing neither spoke once. When he has friends over, he warms up and chatters, and at school will make a comment or two to that child.
That said, DD had a girl who never spoke in class, and when called upon would tear up and eventually whisper the answer in the teacher's ear. By the end of the year she was chatty with the other girls, even though got stage fright when called on in class. She's in kindergarten now and I ran into her at Chuck E Cheese and after politely answering my badgering questions about kindergarten, she grabbed my daughter's hand and said "let's go Alex!"
So it may just take ALOT of time. But pre-k is about learning the social skills she needs for school, and with 1.5 years more of pre-K, I bet it will come at her pace. Good luck!