Social anxiety is the 'fear of social situations that involve interaction with other people, it is the fear and anxiety of being negatively judged and evaluated by other people'. Sometimes, social anxiety will negatively affect the hobbies or clubs someone is part of outside of school.
When you join a club/ team voluntarily the imapcts of social anxiety may seem worse. I joined army cadets almost two years ago now (September 2015). At the start, we did not have to do anything that involved public speaking or taking parade. It was easy, and any time we had to shout numbers I was nervous and quiet. Recently, I had to do a 3* drill test (which involved taking parade and shouting) and it would take me ages to even build up the courage to say the commands. Any pressure put on me would make it worse- I cried the first two times i had to take parade. Over time i eventually built up the courage to take the parade, but still lacked the confidence to do it without developing any mannerisms. I would rock on my feet, clench my fists or go to cough or laugh. I failed my drill test in the end- because i still didn't have the 'confidence' or 'conviction' to shout in front of that many people, and the stress had gotten to me. Having social anxiety, or anxiety in any form, can be extremely hard to deal with- especialy if it restricts your activities outside of school.
For me, to get over my anxiety of speaking in front of people i had to practice and practice- and even though i would improve each week it would take me time to get to a point where I could say what i needed to. To get over social anxiety in a hobby, it may just take time. You would need to remember why you joined in the first place, and why you love doing it in the first place. Overcoming it may be different for each person, like each person's anxiety is different for them. You can get through this, the people around you will help and ease some of the pressure off of you.