AIUI it is not possible to password-protect an album within WPPA+ itself.
I got around that by using the [free] WP-membership plugin in combination with its redirect extension, combined with the “encrypted photolinks” option in WPPA, as described at https://www.ads-software.com/support/topic/wishlist-different-albums-for-different-users/#post-11555776. I make up different usernames and passwords for different people, and they get to see a different selection of albums/pictures, depending on their “membership level”.
Happy to help you out if this were to fit the bill for you, all I can say is that this setup works very well for me, even if its operation is a wee involved.
Always remember that if you really want to keep a picture *private* don’t upload it to the internet in the first place!
serge
