Once you have a group set up, you can start adding accounts to it. To do this, click the row you would like to assign to a group (in the 'Personalise Accounts' section) to get the following modal.
From the example above, you'll see that there is an option for assigning the account to a group and the dropdown will contain all groups that you have created. If you have selected a 'cash in' account only 'cash in' groups will be displayed and vice versa. Once you update the account it will show as part of a group in 'Personalise Accounts', similar to below.
Also in the main cash flow table.
The group can be expanded to show the contained accounts by simply clicking on the row in the cash flow table.
If you would like to edit the group, find out how here.