Hi.
I love the command line. If you are after just the domain usernames, log onto a workstation thats on the domain (you can even do this on the server as well). Make sure that the user account you login with has got rights on the domain .. nothing special .. just normal user rights will do
1. go to the command line (Start then Run and then type cmd and press ok)
2. At the command prompt (the black screen) type the following
net user /domain > c:\usernames.txt
3. The above command queries your DC and gets the usernames of all users in your AD and pipes it to c:\usernames.txt. You can then open this file and have a look at it.
Hope the above helps (I assumed you are not aware how to get to the command line .. apologies if you already know and found my directions offensive)