Give her time....my daughter was just like this. I will share with you some things I did. But I also wanted to share, my daughter barely talked to any of the other kids in her extracurricular activities either, but this year (she is 7) its like she is totally coming out of her shell and on her own too. I think its just because of experience and time.
But one of the main things I did as the years went on is play dates and more play dates. With different friends too.
I started with just one play date with a certain friend only to find out my daughter used this friend as a crutch and then this friend would leave my daughter behind. So at that point I planned a play date with another friend, then another. And now play dates aren't as important but my daughter feels way more comfortable because she knows quite a few friends relatively well. These play dates were all on me too...I had to search out the parents and plan the play dates but the parents were very receptive.
Other than that try not to worry so much about her playing with herself unless it is truly bothering her. If its just you that is concerned then let her be. Believe me in time she will open up. And if you think she needs help, plan those play dates! Good luck!