If the Stored Process is generating the report element (such as a crosstab), then using the 'by' statement within that PROC will generate STP results for each BY group.
If you would prefer to get the Web Report Studio Group By breaks, I would recommended that a SAS Information Map is generated to execute the SAS Stored Process, then in Web Report Studio create a report using the included report objects and add the Group By breaks there.
A word of warning: these are instructions for group breaks that are defined on a report section in WRS with a specific information map data source. You can still use the distribute report wizard on a report containing only a stored process. HOWEVER, if you had some sort of code in your stored process which defined groups, you'd have to write more code to distribute by those groups, probably using FILENAME EMAIL statements. So, the easy distribution for group breaks only works if you built the report based on an information map in WRS and selected hierarchy or category data items for the group breaks.