Like the others, I am stunned at Coache's attitude... she should be telling you what/why/how... Are you able to approach her and say

..."y'know the other day out riding, I know I didn't have everything quite right - can you give me a few pointers... particurlarly when you said **** when I was riding beside you..."

Hopefully she'll see her roll as mentor and model and support your learning.

It is very important to ride at a consistent pace and in a straight, predictable way when with a group
I'm sure if you explain to everyone that you are learning about riding in a bunch and you want to stay at the back rather than be in the middle or at the front, they will all be very happy for you to sit on the back until you have gained confidence to take a turn at the front

Good on you for not being turned off completely by this womans grumpyness... perservere, riding with others is different from solo and offroad, and is a lot of fun.