Define all the group events, i.e. classes by going to Administration > Class Types and clicking on the Plus-icon in the top bar of the screen and adding all your class types to the list. You can also write a description of your class that will be visible to your customers as well as define the default color per class. The color will be visible in both your admin calendar as well as your customers' side. You can edit these settings at any time by selecting the class type you'd like to edit and after that the Pen-icon in the top bar to enable editing or the Trashbin icon in order to delete a particular class type.
You will also need to define a Resource for the Class as well one or several Workers that can work as instructors for the class.
You can define the resource by selecting the Resources tab next to the Information tab and then clicking on the Pen-icon to be able to select available Resources (please note that you will need to have defined Resources before this though from Administartion > Resources). After defining your resources you'll also need to define your Workers in the same way, by selecting the Workers tab next to the Resources tab. You now have a new class that you can add to your calendar at any time, by going to the Calendar view (from the main menu), clicking on an available time slot and creating a new class.