If it were me and I felt comfortable doing it, I would go over the aspects of my life that were most important to me. Things that formed my current personality and affect the life choices I've made. That may or may not include trauma. You could mention that there was a traumatic experience, but not go into any details if you are not comfortable doing so. However, that could open the door to being more comfortable in sharing it later.

I think you are courageous just being in group therapy. The thought of sharing with a lot of other people is frightening. Since I started this round of therapy, I am more comfortable sharing portions of my life, but one-on-one with other people. For months, my T kept telling me that I was going to share my story in a "public setting". I'm not so sure about that.