Within the Assisted Setup for creating a Promotion Card, there is also an option for creating the Promotion Group Codes. This button is seen throughout the Assisted setup screens, but is greyed out (not available) unless you select the following combination: Criteria Type = Promotion Code and Promotion Code Type = Group
Like the manual setup, you will be presented with a Promotion Code Group card, where you can then fill out the relevant details.
From this screen you can choose to generate the codes manually or use the Generate Codes option.
NB: If you do not fill in the Code Format field on the Promotion Group code card and try to generate codes it will error as there is no code format for it to use.