Yes, helmets are a necessity at TR. Although you are free to do as you wish, the first time I went to TR I wondered if I should bring a helmet or not, I did thinking it was better to be safe than sorry. I was extremely happy that I brought one, as there were a few instances where I nearly bashed my head into a tree. I'm sure that somebody can lend you one, the only extra helmet that I have is my old BMX helmet, so I'll toss that in my car in case you need it. If somebody else has a helmet that they would be willing to volunteer, let us know.
Welcome to the boards sanders!