Add or remove shared files and folders in iCloud Drive on iCloud.com
After you have shared a folder, you can add files and folders to it or remove files and folders from it. All participants can access the files you add, but lose access to the ones you remove. Participants who are allowed to edit the shared files can also add and remove files.
Add a file or folder to a shared folder
In iCloud Drive on iCloud.com, drag the item to the shared folder.
If you place a file that is already shared with others into a shared folder, only the participants of the shared folder can view or edit it. Participants who previously had access to the file lose their access (unless they also have access to the shared folder). If you do not own a shared file, you cannot move it into a shared folder.
Remove a file or folder from a shared folder
In iCloud Drive on iCloud.com, drag the item out of the shared folder.
All the participants of the shared folder lose access to the removed item.