In the „Management” Module, at the subcategory „Groups of children”, you will find 2 types of groups: Principal Groups and Optional Groups. In the upper right part, press „Add group” button.
There you will see a new field to enter the name of the group and you will be able to select the type of the group and a colour for it. Save your changes by pressing „Add” button.
The difference between the principal group and the optional one is that a child cannot be a member of an optional group, without being in a principal group already. The optional classes selected by the parents are part of optional groups.