We do not provide direct support for Active Directory, but with the help of the Site Integration Package, you can do it the following way:
Basically, using the SIP, you can perform any kind of external registration without too much of a trouble. The software cannot authenticate members without keeping a record in its own database, so to perform external member db authentication, you will need to use both the PGDRegistration and the PGDLogin classes.
The following is an idea on how you can authenticate users with Active Directory (NT domain/Windows Authentication)
First of all, make sure that you disable Anonymous access to the forum in the IIS manager.
Once they use the challenge/response prompt to login, the server variable, logon_user, will contain his logon name, and you can check whether this logon name exists in your db. If not, use PGDRegistration to insert a record into the database, and if it exists, use PGDLogin to perform login.
When you insert a record into the database w/ PGDRegistration, you need to come up with a password for each member. You can just make it 12345 (so that everyone uses the same password), and use this same password to perform external login. To wrap things up, you will have to remove the forum's login, registration, password reminder, and the change password pages in order to have a complete integration with Active Directory (just delete those pages and then remove the link to them in your header).