You experienced one of the pitfalls of a group ride. I love group rides mind you, but they make me nervous when I get into traffic. From a group ride standpoint, you need to have a chat before the ride to talk about how you are going to deal with someone approaching from the rear in a situation like that. When I ride alone, I assume they will turn and slow in preparation. You should also have a plan for what you will do when a potential left turner is oncoming. Group rides can get ugly when you have to take emergency evasive action. We get in group rides, put our heads down and grind. We don't like to slow. I have found when I ride alone I have much better situational awareness.