Ususally in a group of people, I am not quite willing to talk much, and I can't do so. Perhaps, I am not used to talking to a group of people, or I find it less comfortable to do so. Or I am not interested in those common topics that they're discussing. In a sharp contrast, I find myself much talkative when talking to one single person only. How should I tackle this problem?

conversate in a way so everyone is involved, make eye contact with the other people as wel. if you make eye contact enough they will start listening because its rude to not listen when someones talking to you.
you can ask questions and ask others what they think about it.

1. make gestures to more people
2. adopt correct open body posture to whole group
3. eye contact to every person
4 talkative mode : ask questions and ask opinions , involve other people give them a chance to give their opinion

if you talk to more people then other people will get interested and listen along.