How do I restore/"un-delete" a deleted group?
On the web: Click your name in the top-right and go to "Your account". Then scroll down to the "Advanced features" section, click on "Undelete a group", and then select the deleted group from the list that appears.
On iPhone or Android: In your "Recent activity" feed, tap on the entry for when the group was originally deleted. The app will ask you if you'd like to undelete your group.
On iPhone or Android: In your "Recent activity" feed, tap on the entry for when the group was originally deleted. The app will ask you if you'd like to undelete your group.