With Reporting Groups created, you can now filter groups in Reporting to only display courses completed by learners in those groups. To do so go to Reporting and under "Select Reporting Scope" click on Change.
You will see all Reporting Groups in your site. Select the groups you'd like to see, then click on OK.
All of Reporting will now display results of learners in that group. Under the Reporting Scope you should now see the group's name. If multiple are selected it will say "Multiple Groups Selected."