I needed to add an existing class (.dll) to a project and for some reason it took many articles before one of them mentioned how to get it added to your project or website. It's actually very easy to add. Just follow along below:

From within Visual Studio 2008, open your current website.
Right-click the top entry in the Solutions Explorer (normally your site root) and choose: Add Reference...
On the .NET tab, find the System.DirectoryServices entry, select it, then press the OK button.

That's it! Now all you have to do is figure out how to use the Class.