Left-click the column header (it highlights blue) and drag your mouse across the columns you want to include in the group. Release your mouse and click Group Periods. The periods you selected are grouped together and the values you now see in the group column are the total of the individual column values. The column header identifies the grouped columns. In this collapsed state you cannot edit the values. Cells are white.
Click the group header to expand the group and view the individual columns again. You can edit the values as usual.
Collapse the group ability to spread you are limited to the columns within that group.
As a result of these changes, the way you sort solumns has changed. Right click to view the sort options. You also get the option to ungroup