Private projects are a premium feature which is available for Teams.
To make the existing project private, go to the Admin tab within the project:
From here, navigate to the permissions tab and scroll down to the bottom of the page where you will see the option to switch this project to private:
The only Team Members who can view and access this Private Project are the Team Members that the Project Admin invites.
To see all benefits of upgrading your Project to Premium, click here.