If you are using Group Labels in your document and need to move slides, there are a few things you need to know. A slide can be added to another group or between groups.
When dragging a slide, you'll see one of the three indicators below. The first shows adding the slide to the end of a group. The second shows moving the slide between two groups. And the third shows adding the slide to the beginning of a group. If you are moving a slide between slides in a group, you'll see the same indicator as the second image below. If you hold the Option key while dragging a slide, it will duplicate the slide where you release it. When you move a slide, the existing group changes to the one you add the slide to or to an unassigned group if dropping between groups.