Update: A better explanation of what I answered can be found here. Please use that information instead.
ORIGINAL: macOS doesn't have the same unix commands as Linux. To add a user, properly use the system preferences pane for Users & Groups.
System Preferences will have a Users & Groups section. There you can click the + icon in the bottom left to add a user as standard or administrator.
macOS uses a directory service to administrate users. The
adduser command equivalent for Mac is:
dscl . -create /User/USERNAME_HERE
You may need admin privledges, therefore
sudo will be needed like so
sudo dscl . -create /User/USERNAME_HERE
Followed by a long chain of commands to set up that user properly. Please see the manual page for more detailed information.