Go to the Filter Tab on the Report.

Click on the lookup or ellipse button next to the Client-Matter Number.

Highlight/select the first Client-Matter Number on the Client you want to include, hold down your shift key, and highlight/select the last Client-Matter Number on the Client you want to include.

Right-click and choose Mark Selected. 

This will mark all of the Matters on the Client.

Proceed to the next Client Number.

This will allow you to select a group of Client Numbers quickly and produce a total for those that you selected.