I do some night training in the winter, plus, a lot of the group rides seem to come in after sundown this time of year. I suggest hooking up with a friend and use the buddy system though. I was out with a group last Wednesday, it was getting dark and we were still 20 miles out. I had a light and we made our way to a bike path that would take us to the shop. We came across a lone roller blader who had crashed. He was totally unconscious, laying face down in a pool of blood. His face seemed to be fractured and he was having trouble breathing. Took about 30 min. before the emergency personnel found us and this guy was knocked out for about 20 min. of that. Who knows how long he'd been there before we found him. Even when he came around some, he was unresponsive. It was pretty freaky, I hope he'll be OK.
So I'm just saying, having a buddy along could be a good thing. I'll still go out solo, but I have a teammate that trains at night too and we go together if at all possible.