Question

your friend is shy and often faces problems with group task she/he is unable to give her/him best and gets left out often with your partner discuss?

Answer 1

Your friend is Shy. They miss out in group discussions/tasks. Aren't able to keep up their opinions, aren't able to speak well 'cause of shyness. Right?
 
Tell them it's their loss. Make them understand being shy isn't good. They'll face loads of problems in future, they need to speak up. It won't help being silent, after all. 

Ask them the reason. Mostly the fear of social judgement is the reason for shyness. Are they afraid that someone might make fun of them? Are they afraid they might do a mistake? Or are they simply uninterested to speak up?
 
Shyness can be minimized. Tell them that it's okay, it's fine. They'll overcome their shyness well. Ask them to start interacting, by saying 'Hey's and Hello's to anyone they meet. Tell them to prepare well before speaking.

Everyone has to face challenges. Even I was quite shy at sometime, I still somewhat am. Tell them to take it as a challenge, make them believe that they'll for sure complete it.