Like you I am pretty shy, but I'm a loner by choice because of being burned several times by supposed "friends". I went to a two day class by myself and didn't know a soul. I stayed in the local motel and it was so relaxing. I got tons done and really enjoyed myself. The gal who sat next to me at the class was friendly and invited me to join her for lunch both days. We really hit it off and both of us enjoyed ourselves.
I went to our guild retreat last year and had a terrible time. I knew everyone, it was very cliquey. I seriously considered just leaving, but knew I had to see those people again, so I didn't. I will never go again.
Bottom line I would sign up for one and see what you think. Go and be friendly. Maybe pick someone who looks like a welcoming person to sit by. If the situation is too uncomfortable move places, work on your own projects or leave early. I've made myself learn to be comfortable being with me, so I would take a chance.