This feature is only available to Enterprise users.
The first step is to click on the name of the stage. A sidebar will appear.
In the field “Phase Details”, you can see that there is a “Privacy” section. Click on the pen icon in the top right corner of the field.
Once you have clicked on the icon, you will be able to put a check-mark in the checkbox of the “Privacy” section. Press “Update”.
Once you have done this, in the field below you will be able to select which team members can access the private stage. Click on the pen icon in the top right “Members” field. From a drop-down menu you can select the members. Press “Update” once you are finished.