You need to edit /etc/profile for that user... you can specify the PATH where commands are executed...see file example below.
To execute the command you will need to know where the command is located... /usr/sbin/
You can type the fullname of the command and not worry about editing the /etc/profile however it becomes redudant to continue to remember where all the commands are stored.
[Example]
/usr/sbin/useradd
Hope this helps if you need help on the profile I have copied one below the profile is located in /etc/profile
# System wide environment and startup programs, for login setup
# Functions and aliases go in /etc/bashrc
if [ `id -u` = 0 ]
then
PATH="$PATH:/usr/lib/sf/support/scripts"
else
PATH="$PATH:/usr/lib/sf/support/scripts:/sbin:/usr/sbin"
fi
pathmunge () {
if ! echo $PATH | /bin/egrep -q "(^|

$1($|

" ; then
if [ "$2" = "after" ] ; then
PATH=$PATH:$1
else
PATH=$1:$PATH
fi
fi
}
# Path manipulation