Yobeth,
If this is something that you really want to do (for you and for those who this event serves) yet you don't want to be there and not know anyone then could you ask a friend to volunteer with you? Also, think of this as a chance to meet new people in a pretty non-threatening environment. After all, you will all be there for the same reason. Just smile and say "hi" (trust me, I know how hard this can be) and I bet soon you will "know" some folks. I also think you have to decide if this event is really worth all this angst and anxiety. Maybe it would be better not to volunteer at this time for this event.