Once created, all groups need to be customized before you can send invitations. Groups that are still incomplete will say "Incomplete" next to the group name.
Once you select a group to customize, below is the screen you will be brought to. Always start with "Customize Event Website", even if you don't plan to use an event website for your event.
After customizing each section, don't forget to "Save & Continue" on the bottom right of the page!