Shyness or Rudeness
Sometimes it's easy to confuse the two. A child who is greeted by an adult friend of yours may duck behind you and refuse to say "hello" or to speak at all, even when prompted. But rather than scolding her, encourage her to be polite and greet adults you introduce to her. You don't want her to be comfortable with every stranger; after all, we teach our kids to be wary of strangers, and to expect that she can immediately be friendly to someone she doesn't know sends a contradictory message. But we do want our children to be respectful to others, especially to friends and relatives.
Help her find her voice
Talk it out and see what's really bothering her, but don't speak for her and say, "Oh, she's shy" or "She doesn't talk." Sometimes children just don't know what to say, or how to say it. Sometimes they have real fears and anxieties of strangers. Reassure her that you're there to support and protect her. If she is still reluctant to speak up in unfamiliar situations, practise with her. Pretend that you just met, and act out a conversation with her. "Hello!" "Hello, nice to meet you!" She might feel silly at first, but that's okay. Keep practising and make it fun, so that she can feel more confident about expressing herself. But remember this may take some time, so be patient. By building her confidence, you will give her the courage to use her voice.