In case anyone didn't already know, there's been a way to do it since ULX started using userallow/deny.
Notepad or other text editor, open gmod/data/ulib/users.txt
If you want a slightly more formatted way.
From your server console, run "ulx debuginfo"
In the console, it will tell you where it placed a file.
That file has all sorts of info, including a slightly more formatted users ucl.