How to populate a dropdown list with the usernames from an organizational unit in active directory?

Hello All. 
I found this thread (http://forums.asp.net/t/1651004.aspx) and it appears to be on track with what I would like to accomplish.  However, I need to modify the code to provide the  users within the "Standard Users" OU instead of a list of usergroups.  Any help would be greatly appreciated. I'm not sure it's needed, but here is some additional information: This is for an intranet application I'm using Visual Studio 2013 .net Framework is 4.5.1 Server name is xyz.org search base is DC=xyz,DC=org Port Number is 389 Username is user1 Password is password (not really, but you get the idea)

Here is the code from the thread referenced above:
public ArrayList Groups() { ArrayList groups = new ArrayList(); foreach (System.Security.Principal.IdentityReference group in System.Web.HttpCont ...