Take a look at PiwigoPiwigo, open source gallery software for the web and available on http://piwigo.org.
Events Based Photo Sharing: you can group your photos by album. Piwigo supports several levels of albums (as much as you need).
Private: you can make the whole gallery private or just make a few albums or photos private. User "john" may be permitted to see album "eventA" and "eventB" and user "paul" can only see "eventC". The webmaster decides.
Downloadale images: Piwigo let the visitor download photo one by one. If you want to download several at once, install and activate plugin Batch Downloader.
Comments/Description: core features.
Easy upload: web upload, FTP and applications like pLoader, Lightroom, iPhoto, Aperture, shotwell, digiKam. Applications for iOS and Android.
Active Directory: a plugin for LDAP is under preparation, I think it will be compatible with Active Directory.
More than 200 extensions are currently available for Piwigo 2.4