To find the DN run the command dsquery group -name WebSiteUsers
If you have a domain controller set up for PowerShell (you should; it's awesome) you can run the command $WebSiteUsers = Get-ADUser -Filter | foreach
Might not hurt to add a -WhatIf on the Add-ADGroupMember command to double check it's going to do what's intended.
You can also get this list using the Active Directory Users and Computer snap-in. You'll need RSAT installed to do this from your workstation, otherwise you can remote in to a domain controller and open it.
Right click on Saved Queries and select New, Query:
Give it an abitrary name and a short description, then click Define Query:
Under Find:
select Custom Search
. Click on Field and select User, Member Of
Enter the name of the group you'd like to include and click Add:
Now you can view this list in ADUC. To export it, click the Export List button. This will output to a tab delimited text file.