Social ride etiquette

B. You are at the back of the group riding along a single lane road. There is a driver behind. Is it better to yell 'Car Back' and move left to let the driver past, or to move right and prevent the driver passing with a wave and a smile?
Warn the group ve7rbally and move left. Moving right may annoy the driver, they sometimes get irate and start beeping and/or try to force their way past. Whereas if they see you've acknowledged their presence and made the effort to move left, they usually wait patiently.

I think it is only appropriate to move left when you are sure the driver can clear the whole group passing wide, which isn't often in urban environments. In most cases its better to move right and keep them back with a thumbs up. This is a major issue where its better to tolerate a beep and even annoy a driver for a short time than to let them pass where they may pass close to someone. In many cases its better to encourage people to ride 2 a breast.