Any work created for a private or hidden contact will have the same privacy level as the contact. All members of the client team will automatically have access to any work created for that client. In addition, the work item assignee, members of the work team and any other colleagues with assigned tasks, notes or emails within that work item will also have access to the private or hidden work item.
There are several ways to add a colleague outside of the client team to a work item for a private contact:
Assign the colleague a task within the work item
Assign the colleague a note that is linked to the work item
@mention a colleague in a note that is linked to the work item
Add the colleague to the associated client team
Reassign the work item to the colleague
Assign the colleague an email that is linked to the work item
Only those with access to the work item, organization or contact will be able to link and unlink a note or email conversation to a timeline for a private or hidden work item, organization or contact.