How to generate top 10 users mail activity report?
Perform the following steps to generate top 10 users mail activity Report:
Select Exchange Online Reports from the available vertical tabs. Then from left pane of application, select Distribution Group Activity.
On the selection of "Top 10 Users Mail Activity" from the list of available reports, a dialog is displayed as shown below:

On selecting the distribution group and date range, the resultant data are displayed in a window as shown below:
The chart view represents the pictorial representation of the report. On selecting the chart view tab, report is displayed as shown below