Be prepared to accept that your thoughts, feelings and attitudes towards food are skewed/wrong, and make good use of the group sessions to explore the reasons why and change your thinking patterns.
As BL said, there is NO reason at all to eat on this diet, and believe me if you do, it will make it 100000x harder to finish it!
Don't be afraid of photographs - take some yourself before you start, and take one every week, you'll be suprised at changes, they are much more visible in a photo than they are in the mirror.