Click the "Groupings" tab (found near the top of the page)
Find the grouping you would like to populate with groups
Over to the right there is a little icon for viewing groups in grouping. Click this.
Choose the group (or groups) you want in this grouping from the list on the right and click "Add"
When you are done, you can click the "Return to groupings" button or just return to your course. You do not need to save changes.
Removing groups follows the same process, but instead of selecting the group from the right hand list and clicking "Add", you select it from the left hand list and click "Remove".