Open the dropdown menu next to the album on the side menu and select Customize Album. You can also customize the album by using the Customize Album button at the top of the Album page.
In the customization options, click the option Enable Password Protection and enter a password for your album.
When users try to view the album, they will be asked for a password. Share the password only with people you want to view the album.
If the password protected album is a parent album to sub-albums, these sub-albums will also be password protected with the same password unless they are individually overwritten. For example, each sub-album within the same parent album could be assigned a different password.
Adding Password Protection to a Shared Album
You can also add password protection to a specific share link. From the Organizer, click Share Album and then click on Add Privacy.
In the Privacy window, enable Password Protection and add the password.
In this case, only those who access the album through this particular share link will be asked for the password.
Adding Password Protection to Album Downloads
You can permit users you share your album with to download it. Add password protection to this feature to maintain control of who is downloading it.
When the Enable album download is activated, you will also be able to set password protection.
For details on how to further protect your albums, see the article How to Share Albums Securely.