Trying to restore an Azure Active Directory group may result in an error similar to the following logged in the Events:
"The group <groupName> was deleted. Please specify the option to restore groups from the Recycle Bin"
Selecting the option, 'Restore deleted objects from Recycle Bin' and trying the restore again results in the group not being restored
In addition to selecting the option 'Restore deleted objects from Recycle Bin' you also need to select the option 'If an object is not found in Recycle Bin, create a new one'. This will result in the group being recreated and the membership restored from the backup.
Please note: The creation of a new group results in a new object ID. This is required as Azure hard deletes Azure AD groups when they are deleted.