Please go Maya!! Coincidentally, we just had our museum volunteer dinner last weekend. I know you don't feel you know anyone, but I bet you will by the end of the evening.
Maybe you could confide in the person you feel closest to there (maybe your new boss)... "I'm going to go, but honestly I'm a little nervous that I won't know anyone!". Hopefully he or she will take the hint and either stick by you or put you in good hands.
I find it helpful to ask people about themselves, "So, are you a volunteer? Which area are you in?". I'm shy also, but I've found there's rarely an awkward moment if you just make an effort at a little conversation. Many people will be just as nervous as you and will be busy worrying about themselves! You don't have to be a rock star networker at this thing, just go and see where the evening takes you.
Push yourself and go, it's a little painful but the best way to grow. You never know what could come out of it!