all children are different. you have to find what works for her. maybe she is just "slow to warm up" around others. try role-playing with her with dolls. pretend you are a new girl at school and your daughter her classmate who can share with her many new things. i think many social skills can be taught with playing with barbies or dolls...just make it fun. she may still be shy around others, but at least if she wanted to talk to some other kid, she wouldn't be speechless. she would remember phrases you said to introduce and get to know someone new, from the games you played with her with the dolls.
also, playing with the dolls you could help teach her about diversities, handy-caps of others, religions, illnesses. maybe you could help reduce any fear she may have around people who are different than her.

my problem as a kid was thinking i was different than others and it was a bad thing or they were bad because they were different. it would of helped me a lot to learn early in life that differences are beautiful.

<font color=purple> But a stranger in a strange land, he is no one:
men know him not and to know not is to care not for.
Bram Stoker, Dracula, 1897 </font color=purple>