Open the Powershell shell (pun intended) on your domain controller and run the following command:
This will list all members of the 'groupname' group in your shell. List pipe it into a text file instead:
Get-ADGroupMember 'groupname' > list.txt
We don't need those unecessary columns though, so let's just list the NAME column and nothing else:
Get-ADGroupMember 'groupname' | select name > list.txt
Run the shell as admin...