I don't think being introverted is a good reason to not participate in discussion. I'm not sure if this is the official definition, but my understanding of introverted vs extroverted is that introverts get their energy from themselves whereas extroverts get theirs from others. It makes me think of introverts having some quality alone time to compose themselves and re-energize, and extroverts hanging out with others and feeding off the crowd energy and spirit to get their mojo. (Blogger doesn't think mojo is a word. What's up with that?!)
      I do believe I'm introverted. But sometimes I feel really energetic and talkative. It depends on my mood. I'm not likely to be super chatty with someone when I first meet them, but that's just how I am. Once I get to know you you'll probably have to tell me to shut up at one point or another. I may not contribute to the discussion much in Scholars, but that's because most of the time we've been talking about politics, and my last post describes how I feel about that. But I contribute in Psychology because that's a subject I find interesting. Sometimes I talk just because I hate it when a teacher asks a legitimate discussion question and everyone just sits there staring blankly back.
     Even if you feel more comfortable hanging solo, you can still contribute to the conversation. And often times I've seen the most bubbly, outgoing people become completely mute during class discussions. Sometimes it's not about how talkative you are; it's about how much you actually care about contributing.
    Why do we spend all this time classifying people by being introverted or extroverted? Isn't that just generalizing? Aren't we supposed to get to know people for who they really are? It's easy to just assume that someone is quiet during the class discussion because they're introverted, but that doesn't make it true. Maybe that person just doesn't have anything interesting to add to the conversation. Maybe they just don't care about the topic. Maybe they want to listen to others' thoughts. Maybe they didn't do the reading that the discussion's over. I think those are more likely reasons than just being introverted.
     But if you are too quiet to feel comfortable talking in a discussion, I guess my advise is to listen to what others are saying and start off by agreeing or disagreeing. Personally, nothing gets me into a conversation more than hearing someone say something that sounds dumb and or completely opposite to my beliefs. But maybe that's just because I'm an argumentative, stubborn person.