The Group By drop down list options are Date, Resource, and None. The default is Resource.
Group By Date: Resources chosen from the Resources drop down list are displayed side by side, below each date that has been selected in the Calendar navigation area.
Group By Resource: Tasks are grouped first by the resources selected from the Resources drop down list, then by the date or dates selected in the Calendar navigation area. This is the default Group By view whenever the schedule is first opened.
When using three or more Selected Resources, three will show in the window:
To show all, click the + at bottom left, then save the current view:
Group By None: Dates chosen in the Calendar navigation area are displayed side by side. Tasks for resources chosen from the Resources drop down list are grouped together, below each Date header.
Further information