Hi @Richard_Conway_Tri_G and @Jules and welcome to the Zwift forums!
I have never tried to do a workout while also doing a group ride, however I have done a workout during a meetup. Are you confusing the group ride and group workout maybe? They are different events. If it was supposed to be a group workout then you should have all been getting the exact same intervals and instructions on screen. Group rides are just that, group rides, no intervals or set workouts.
Additionally, what trainer are you using? Is it a smart trainer that will control the resistance or a classic “dumb” trainer where you have to change gears manually to increase/decrease resistance? The more info you can provide the better. Including what you are running Zwift on (windows 10, apple tv, etc…) and how you are connecting the sensors/trainer (ANT+ or bluetooth).