If someone in the group has Zotero open but isn't using auto-sync, they can copy the items in that collection to another collection and then sync. The first collection will be deleted but the second one will sync up to the online library.
If someone in the group doesn't have Zotero open but does have auto-sync enabled, they can temporarily disable their internet connection and then perform the above steps.
If everyone has already synced the deletion, someone can trying restoring from the last automatic backup.
Upgrade Storage
If someone in the group doesn't have Zotero open but does have auto-sync enabled, they can temporarily disable their internet connection and then perform the above steps.
If everyone has already synced the deletion, someone can trying restoring from the last automatic backup.