If the EcoSys report designer has enabled this option, the Group tab will be accessible. When configuring the report, on the Group tab the designer may choose a column to be used as a default grouping. Additionally, on a column by column basis, the designer may check the User Can Group By check box which will give the user the ability to make grouping changes at report runtime.
Columns enabled for runtime grouping will display in the Group & Sort tab at runtime. These tabs are seen in the screenshot.