I have gone to a retreat alone when my friend had to change plans. I did know a lot of the women casually through guild. I think sewing next to someone is a good way to get to break the ice. I also listened to a book on tape the third day (while I was sewing) when my sewing enthusiasm lagged.
So I agree with the above suggestions. Introduce yourself as new to the retreat but not to quilting. I think Leah Day and the ladies in the modern quilt movement have made women more aware of a younger generation being involved in quilting. Bring something to listen to, and snacks. There was a lot of snack sharing at the retreat I went to and people walking around asking what you are working on but I am no expert.