Shell Programming and Scripting

#!/bin/bash
trap 'exit 0' 1 2 3 15
printf "Username: "
read touser
trap - 1 2 3 15
if [ "$touser" = "new" ]
then
    echo "Create a new user account here"
else
    exec su - $touser
fi

while [ -z "$NEWUSER" ]
do
        printf "New username: " >&2
        read NEWUSER || exit 1
        if ! echo "$NEWUSER" | grep "^[a-z_][a-z0-9_-]*$" > /dev/null
        then
                echo "Usernames must be a-z_-" >&2
                NEWUSER=""
                continue
        fi

        if grep "^${NEWUSER}:" /etc/passwd > /dev/null
        then
                echo "${NEWUSER} already exists" >&2
                NEWUSER=""
                continue
        fi
done

exec sudo /usr/local/sbin/newuser.sh "$NEWUSER"

#!/bin/sh

useradd "$1"

passwd "$1" || userdel "$1"

new ALL=NOPASSWD:  /usr/local/sbin/newuser.sh

...Then I "man pw", and found the usage of (pw) user add is a little different from the standard ones: 
pw [-V etcdir] useradd [name|uid] [-C config] [-q] [-n name] [-u uid]
   [-c comment] [-d dir] [-e date] [-p date] [-g group] [-G grouplist]
   [-m] [-k dir] [-w method] [-s shell] [-o] [-L class] [-h fd | -H fd]
   [-N] [-P] [-Y]

In FreeBSD, it should be "pw useradd username -g groupname", and not "pw useradd -g groupname username".