You can delete a group album by leaving the group album and then removing all your photos when you go. Empty group albums will be removed by the system. Instructions for leaving a space:
MOBILE: Go into the group album you want to delete and tap on the "Settings" icon (this is a gear/cog on iPhone, and an icon with settings sliders on Android). Tap the red "Leave this Group Album" button at the bottom of the "Settings" screen. You will be prompted "Really leave this group album?", tap the red "Leave this Group Album" again. Here you will be prompted "Leave your photos for other members of this group album to view?" Select "Delete My Photos".
WEB: Click the "Settings" button below the cover banner at the top of the screen. This will open a pop-up with information about this group album. Scroll to the bottom of the pop-up and click "Leave This Group Album". It will then prompt you, "Are you sure you want to leave this group album? There is no undo." Click "Leave Group Album". When asked if you would like to leave your photos for others to view, click "Remove My Photos".