Undelete a group

When you are looking at the groups, by default it will be the Group Account view.  Swap to Folder View.

On the report click the kebab menu (3 vertical dots), and where it says Report: All Folder Entities click Change

Select All Folder Entities (with Deleted). 

Then you will be able to see anything that was deleted and you can right click on it to Undelete the group.