I would start with following Online Help Guide on specific content pieces and setting permissions for them: Set Permissions on Individual Content Resources - Tableau
Another option and suggested method of Tableau would be to set up Nested Projects in Tableau Online. Controlling access to the workbooks based on the project they are in. To create a nested project, navigate into a project, select the project tab within the project, then select "New Project". (See scree shot below)
More information on nested projects can be found here: Add Projects and Move Content Into Them - Tableau
Hope this helps!