This is an issue we reacently discovered with adding users through the default settings/users.txt file (
bug report). We have been considering a rewrite of the admin system, and so have not gotten to fixing this problem yet.
Though what you have written there as your command is the exact reason we included the "ulx usermanagementhelp" requirement. The correct command for adding a user is "ulx adduser <name> <group> [<immunity>]".
To get this to work you will need to know your rcon password (this is basic server management 101,
Google if you don't know how to use it), or have direct access to the server's console.
1) run "rcon ulx usermanagementhelp" and
read through the instructions.
2) add yourself as a superadmin
following the instructions from "ulx usermanagementhelp" and running the command as
server console (rcon).
3) run "ulx usermanagementhelp" from your
client console (read through it again just to be safe)
4) add your other users