It appears that all of the canned reports are all-or-nothing, and that you have to control the scope of the reports by selecting either individual clients, save sets, Groups, or Pools. There doesn't appear to be a method for selecting specific clients (which may or may not span Groups and or Pools) to report on. I've also tried creating a new report, but there's no selection criteria outside of the node tree. You can try to create a node that includes just the clients you want to report on. I've not played around with it enough to have been able to successfully do this yet.

Using Report Editor (Tools--->Report Editor) you could try to modify the report (I believe it is "Backup Save Set Summary" report) to read a CSV file that you create containing just the groups you want. To create your own report from one of the canned reports open the Report Editor, make a copy of one of the canned reports (in Report Editor-->File-->Open Copy) This usually gives the opened copy the same name but starting with "Copy of". Once the copy is opened you can edit it. You will see balloons called "Backup Statistics". Those collect the save set info from NetWorker and are called Sources. Instead you can change them to a Source type of "Read CVS" (Once the copy of report is opened, click on Edit, That will open the "Report Item Properties, select Sources-->External-->READCSVFile). That reads from a CVS file that you create with the info in them that you want. There are many Source types and Operators in the Report Editor that might do it other ways too. Granted not an easy task, especially if not familiar with Report Editor" but hopefully gives you more ideas.

In the report definition, have you tried setting the "conditions" of the source to be "Value Type" = variable and "Variable Name" - client. That way the report will prompt you for a client on which to run the report.