How to be more active in a group

Recently, I am doing the certain training of 15 days for CA finals. Everybody here had formed their group. I am also a part of the group, but the problem is I suck at group conversation. I am an introvert and little bit reserved. I can speak well to people individually or when there are less people in a group. But when, it comes to15-20 or more in a group, I am so quite. And, nobody asked about me or include me in a talk because of my weak communication skills. One day I was absent and next day when I came nobody asked about why I was absent?. I feel so alone and lonely. What to do??