Though the question refers to this as a group-wide phenomenon, I've dealt with almost every campaign group I start at least somewhat: There are always a few quiet folks who don't engage with their characters or the party. There are many different reasons for this behavior - it isn't always "shyness". I don't usually bother to dig into the reasoning for the behavior, I just work around it. My favorite technique:
Shine a Narrowing Spotlight
Give each character a special role, goals, and motivation [much like the new D&D Neverwinter Themes]. Tell them each that they need to achieve their goals to succeed. Make the roles/goals/motivations require interaction [need the help of others, need to find something, etc.] This way, when they aren't sure what to do next, there's always something to fall back on - something important to them.
When I ran a Dragonlance module for a group, and Riverwind and Goldmoon were run by a couple new to D&D. She was particularly disengaged, attending because her boyfriend wanted her to. Here's what I did to get her engaged:
They were the last two survivors of their tribe, and they were imprisoned in seperate cels by the main villain - but she wasn't working to escape. So, I made Goldmoon pregnant (and she hadn't told Riverwind yet.)
Her main motivation: Save the baby at all costs, it is the future of the tribe. She must survive and escape.
Result: Near instant deep engagement from the player. Given a clear motivation she now knew how to respond to various events and threats.
When the big baddy delivered a Faustian ultimatum - between sparing her own life or that of her beloved, she responded "I choose to spare myself." I'll always remember the shock on her boyfriend's face...
[They managed to escape, and she delivered the good news which, in turn, influenced Riverwind's role-playing decisions from then forward as he adopted a new role: Future patriarch of a restored tribe.]