You can utilize multiple data sources to create more advanced reporting.
Begin by creating a visualization and adding the initial data source.
For this example we will use Cases created in one of my groups.
Once the filter and source is configured, press "Add this source".
Under Data > Data sources, press "+ Add data source".
Lets now add a source for Cases closed by one of my groups.
We now have overlayed report data from both data sets.
If you still have questions or need additional assistance, please use the 'Contact ServiceNow Team' form.