Grouping is not a Setup switch. It lives in the report builder, and the format changes the moment the first group lands.
- Open the report in edit mode
From the Reports tab, open the report and click Edit. Starting fresh works too, but pick the report type carefully: it decides which fields you can group by.
- Click Outline
The Outline pane holds GROUP ROWS, GROUP COLUMNS and COLUMNS. The first two lists are where all grouping happens.
- Add the group
Under GROUP ROWS, open the Add group picklist and choose a field. Repeat under GROUP COLUMNS if you want a grid rather than a stack of sections.
- Summarise a field, then save and run
Open a numeric column's menu and pick Summarize, then Sum or Average. Subtotals under each group only appear once something is being summarised.
Up to three levels on a summary or joined report, two once it becomes a matrix.
Adding one flips the report to matrix format. Two is the maximum.
On a date group: day, week, month, quarter, year, plus the fiscal versions.
- A matrix report will not let you use the same field as both a row grouping and a column grouping.
- Removing a grouping breaks any custom summary formula that named it, and you only find out when the report runs.