Within Teamwork Projects you can set privacy on any milestones you add to your project so only certain users can access them.
You can set privacy on milestones when first creating them on your project, or by editing an existing milestone.
In the new milestone modal, go to the Privacy & Followers tab, where you will be able to choose who to include.
By default, privacy will be set to Everybody on project.
You can also set privacy to a particular company or companies, some individuals in one company, or some individuals across different companies.
Note: Site administrators in the owner company will always be selected (gray), as their permissions override privacy by default and they will always be able to see the item.
To change the privacy of an existing milestone, hover over the milestone in the project's Milestones tab, and you will see an Edit option.
For more information on adding Milestones see: Adding a milestone