Now that the groups have been set up you can start adding categories to them. To do this click the row you would like to assign to a group (in the Category Manager) to get the following modal.
You'll see above that there is an option for assigning the category to a group and the dropdown will contain all groups that you have created. If you have selected a cash in category only cash in groups will be displayed and vice versa. Once you update the category it will show as part of a group in the Category Manager, similar to below.
Also in the main cash flow table.
The group can be expanded to show the contained categories by simply clicking on the row in the cash flow.