In my local velo club there is a big difference between B and C group rides. Apart from being slower, the C rider tend to have a much wider range of skill, speed and endurance. So you typically end up with loose group riding rather than true paceline riding and a lot of re-grouping, normally at the top of every hill. Avg speeds tend to be 15-16 mph over 30-35 miles and they are normally nice conversational rides.
The B group rides are much faster, typically 18-20+ mph avg and the riders have more similar capabilities. Ths group rides a true paceline with everyone rotating forward to take a pull. There is almost no stopping to regroup as the line stays together except maybe on the steepest hills. It's only conversational during the warm up/cool down parts of the ride.
I have started riding with the B group and much prefer these rides. The frequent long stops on the C rides were irritating and never seemed to get into a rythym like I do on the B rides.