The above posters have mostly covered what I was going to say. I am not outspoken at all and struggled with this sort of thing in the past. What I sometimes did was tell myself to have 'x' minutes of confidence. In other words, for a certain length of time I would speak my mind on a topic, within reason, and I would hesitate as little as possible. It worked in certain situations. You could also discuss with your group members that when you give a hand signal you would like to pick up on a point. However, this could result in hand signals being constantly used so I am uncertain whether it would be completely effective. Otherwise maybe just try and use your English as much as possible prior to the assignment to practice? I hope that it goes well for you!