In some industries, it's commonly believed that remaining silent during business meetings is career suicide. The Wall Street Journal explored Why People Don't Speak Up in Groups to help us understand this phenomenon. It turns out that not speaking up is more common in women and surprisingly, those with higher IQs, according to the study cited in the article.
I'll add a few of my own tips for those hesitant to speak up in meetings. In my experience, speaking up early in the meeting is key. It breaks the ice for you and sets the tone for you to be a contributor in the meeting. The longer you sit there without saying anything, the harder it will become. Before the start of the meeting, chat with the other attendees. Ask them questions about themselves, their opinions, etc. You don't want to overshare, but let people know you a little too. Finally, if the meeting concerns a subject matter that you don't have much expertise in, you can participate by asking thoughtful questions. Yes, questions are once again your friends.
Research shows that more people talk when the size of the group is small. The larger the group size, the smaller the ratio of participants. So if you want to practice being more active in meetings, speaking in a smaller group can be much less daunting. Remember, people probably want to hear your thoughts and questions. You are there to add value. Like everything else in life, the more you practice, the easier it'll get.
On a related note, check out the Harvard Business Review's blog for The Introvert's Guide to Networking, if you need some encouragement in this arena.