Some children are just shy. It's not necessarily a problem. I was (and still am) very shy. I was never bullied at school. I don't suppose anyone noticed me much. Like adults, children come in all shapes and sizes. Some outgoing, some shy, etc. Why should she change. Not everyone can be the life and soul of the party. Some children have a large group of friends, others one or two good ones. Most children, however outgoing are shy with strangers especially when so young. If she is not unhappy then leave her be to grow and develope in her own way at her own speed.
Not everyone needs to "sell themselves" in life. Some are pushy, some are not. We can't all be the same. Everyone has their own place.
P.s. Never could sell myself, always shy and never an extrovert. Am I happy? Yes, wonderful DH, children and DGC. Most content with life as hopefully your DGD will be