Back in 2005 I wrote my first ADSI for plugin WinPE (WinPE 2005 or v1.6), and per request I have since then updated it for every WinPE release… This is the release for WinPE 4.0 (part of ADK).
Update 2013-12-05: For a WinPE 5.0 version, see links further down.
Please note: I don't recommend using ADSI in WinPE. It's not supported by Microsoft, and this release is only intended to support existing solutions using it. Instead you really should use web services to interact with Active Directory from WinPE. Check out http://mdtcustomizations.codeplex.com for ready-made web services for Active Directory, SQL Server, ConfigMgr and more…
Detailed installation instructions can be found in the archive Readme.txt file.
WinPE 1.6 ADSI Plugin
The provided sample script, enumerating users in the Users Container.