I'd apologize to the coach for your etiquette, for missing the sessions, and ask her for more pointers. Our ride leaders often talk through the whats and whys on rides. I bet you could google for "paceline" and find some stuff online, too. Mainly the key is to be predictable and consistent.
I have trouble when I'm riding my heavier bike in a group because it accelerates and decelerates so much differently... if the group slows down, I have to brake my tractor trailer more, and then pedal madly when they accelerate, and it makes for more of an accordion than a pace line. I've learned to take the faster bike even if I would be able to keep up on the sturdy one.