How can I associate customers to groups?

To associate customers to specific customer groups, you can use the 'Groups' panel located in the properties panel, right hand-side colum.

Select the concerned customer and tick the corresponding box(es)

You could also select the lines of the various groups and hit the icon on the toolbar to associate to the selected customers.

To dissociate, untick boxes or hit the other icon on the toolbar once the groups are selected.