In brief, group rides typically have a leader who attempts to keep pace at a specific watts/kg, usually given in the ride description. Other riders are free to go slower or faster. Some rides/leaders are more successful than others at convincing riders to stay together. Group rides are a blast, but they do depend on cooperation from the group.
Group workouts are predefined and based on a percent FTP that changes throughout the ride. You can see a graph of each workout in their description. The nice thing about group workouts is that everyone can ride together regardless of their power output.
A great place to find individual and group workouts is on whatsonzwift.com. Here’s a link:
If you’re looking for general information about Zwift, check out Zwift Insider. My guess is you’ll find information there describing both group rides and group workouts. Here’s a link:
If you want to see what group events are coming up, you can use the Zwift Companion app on your phone. These events include group rides, races, group workouts and other official Zwift events.
Hope this helps!