Chin Huang observes that Microsoft's Active Directory Services Interface (ADSI) exposes a COM interface, so it is accessible through such examples as
# List all users in the domain named "MAIN". package require tcom set domain "MAIN" set collection [::tcom::ref getobject "WinNT://$domain"] $collection -set Filter [list "User"] ::tcom::foreach item $collection { puts [$item Name] }
Also, Franco Violi's TclLDirectory extension is said to understand ADSI.
Yet another way to consider Active Directory is as an instance of LDAP service.