Hello,
the API documentation says:
Allows an owner or editor (if the ACL update policy allows) of a shared folder to add another member.
For the new member to get access to all the functionality for this folder, you will need to call mount_folder on their behalf.
https://www.dropbox.com/developers/documentation/http/documentation#sharing-add_folder_member
If I understand correctly, we cannot add a member to a folder if it is not already shared by the user because we do not have a prior "shared_folder_id"?
Thanks.