When they were launched I thought both Group Workouts and Outside Workouts added very little value but I have used both extensively in the last few months and now think they are hugely valuable.
Group Workouts - my friends and I find these great for tough short intervals (e.g. vo2 max sessions). The ramp test was also great from a motivation and accountability standpoint. We usually extend the cool down for a social catch up too. Really not sure why Zwift is seen as a more social platform when this function allows you real human interaction.
Outside Workouts - in previous years I would drop my training plan for the summer and maybe jump on the trainer once a week, with the rest of my riding outdoors. I thought that I maintained structure but, looking back, I was kidding myself and no wonder that I always plateaued each summer.
This year I have used both features and improved plan compliance hugely. I usually got to 4.0-4.1w/kg each summer but I am now at 4.4w/kg and feel like there is more growth potential.