You can add specific users, departments, or workgroups/projects as a Feed message recipients.
If you add a private (not visible to non-members) workgroup/project to a Feed message recipients list, employees that are not members of this workgroup/project will see the Hidden recipient sign instead of the workgroup/project name.
The author of the message and the workgroup/project members will see the name of the workgroup/project name in the list of recipients:
Read more about workgroups and projects in the article - Projects & workgroups in Bitrix24.