Long press on a group on the main screen, and it will expand. Then you can drag and drop any of the lights of the group outside of the group. The light will then be removed from the group.

You can delete a group by removing all the lights composing it. If there is only one lamp left in the group, the group disappears and that lamp is displayed a “single lamp” again.

Alternatively, you can delete a group by selecting it on the main screen, opening its settings (cog icon) and tapping on “ungroup” at the bottom of the page. Ungrouping will instantly remove all the lights from the group.