Adding a password to one or more specific categories within a gallery can be done as follows:
From the main menu, click Photos » Galleries.
Locate your gallery and click Photos » Manage Categories.
Click on the gear icon for the category, and select Settings.
Click the Required? toggle, and then add the category password.
Click Save.
Note: When a category is password protected, the featured image for that category is blurred so that it cannot be viewed by gallery viewers without the password.