i coach roller derby. I highly recommend it. Very few of the girls I coach could skate when they started. They ALL fall over even still...if they are a good team they will be supportive knowing they were all in your shoes. Give it a go..you won't regret it.
I have some friends in the Gloucester team, Severn Roller Torrent, and, well, what Oracle said really. From what I've heard it is so cool and supportive that you would definitely make new friends even if you did turn out to be rubbish! And there is usually a minimum skating level you have to pass before they let you turn out to fall over in front of paying spectators!
Also I was talking to the youngest member of their team who is 19, and she is quite a quiet and shy girl but reckons she has gained loads of confidence from doing derby, not from getting good at it, just from the atmosphere of it, so yeah give it a go!