Group header and footer rows are not displayed by default for the default grouping, although when you add groups, the table header row is turned on for the new groups. You can toggle these rows on and off in the adorner menu.

To add a group header and footer

Click the table to reveal the adorners.
In the Group section to which you want to add a header and footer, click the expand button. The section expands to reveal the menu.
In the expanded menu, click the drop-down arrow button next to without header or without footer.
Select with header or with footer. The section is added to the table with data automatically appearing formatted in bold.