Adding users to LinuxCNC

First: I'm sorry to ask such a basic question.

I cannot get LinuxCNC to accept adduser or useradd command to work. Is there a tutorial for the tragically unenlightened?

I have a home built CNC using LinuxCNC for about a year, working AWESOME BTW, but have now added a fourth axis and due to the limitations of my software (Vectric Desktop) must change ini file each time I change from XYZ to XAZ. My idea is to have two different logins, one for XYZ and one for the rotary setup. Each Login would use a different setup.

1. Is this a workable idea?
2. Is there a better idea?
3. Why do I smell burning brain cells?

I did as tommylight said to handle two different configurations of my mill. I have two spindles. The standard one and a highspeed 24K spindle. I will never use both at the same time. I just made a copy of my config directory under another name and changed the new config as needed. I also renamed the .ini & .hal in the new directory just for my own sanity.

I have two shortcuts on my desktop, one for each machine. Very easy to run either configuration.

\linuxcnc\configs\SiegX3
\linuxcnc\configs\SiegX3HS

Unfortunately my psychic powers were traded for some magic beans, so I can't work out how you were using the command or what error messages you are getting. Are you trying to create a user without using sudo ?