check Rich's updated version of this at https://gist.github.com/rtrouton/8016797

createmyguest.sh

#!/bin/bash
 
#variables
DSCL="/usr/bin/dscl"
SECURITY="/usr/bin/security"
LOGGER="/usr/bin/logger"
OSREL=`/usr/bin/uname -r | cut -f 1 -d '.'`;

case "${OSREL}" in
	10)
	#Mac OS X Snow Leopard
		${LOGGER} -s -t createmyguest.sh "INFO: executing the 10.6 version of the script"
		${DSCL} . -create /Users/myguest
		${DSCL} . -create /Users/myguest UserShell /bin/bash
		${DSCL} . -create /Users/myguest RealName "Guest Account"
		${DSCL} . -create /Users/myguest UniqueID 201
		${DSCL} . -create /Users/myguest PrimaryGroupID 201
		${DSCL} . -create /Users/myguest NFSHomeDirectory /Users/Guest
		${DSCL} . -create /Users/myguest RecordType dsRecTypeStandard:Users
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_defaultLanguage en
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_guest true
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers__defaultLanguage Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_jpegphoto Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_LinkedIdentity Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_picture Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_UserCertificate Guest
		${DSCL} . -create /Users/myguest AppleMetaNodeLocation /Local/Default
		#setting up an empty password and giving local Kerberos some time to process it
		${DSCL} . -passwd /Users/myguest ''
		sleep 2
		;;
	11)
	#OS X Lion
		${DSCL} . -create /Users/myguest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_defaultLanguage en
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_guest true
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers__defaultLanguage Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_LinkedIdentity Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_UserCertificate Guest
		${DSCL} . -create /Users/myguest AuthenticationHint ''
		${DSCL} . -create /Users/myguest NFSHomeDirectory /Users/Guest
		#setting up an empty password and giving local Kerberos some time to process it
		${DSCL} . -passwd /Users/myguest ''
		sleep 2
		${DSCL} . -create /Users/myguest Picture "/Library/User Pictures/Nature/Leaf.tif"
		${DSCL} . -create /Users/myguest PrimaryGroupID 201
		${DSCL} . -create /Users/myguest RealName "Local Guest"
		${DSCL} . -create /Users/myguest RecordName myguest
		#Lion does not like two users with same UUID so don't use 201 on the next line
		${DSCL} . -create /Users/myguest UniqueID 401
		${DSCL} . -create /Users/myguest UserShell /bin/bash
		#Adding the keychain item that allows Guest to login in 10.7
		${SECURITY} add-generic-password -a myguest -s com.apple.loginwindow.guest-account -D "application password" /Library/Keychains/System.keychain
		;;
	12)
	#OS X Mountain Lion
		${LOGGER} -s -t createmyguest.sh "INFO: executing the 10.8 version of the script"
		${DSCL} . -create /Users/myguest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_defaultLanguage en
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_guest true
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers__defaultLanguage Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_UserCertificate Guest
		${DSCL} . -create /Users/myguest AuthenticationHint ''
		${DSCL} . -create /Users/myguest NFSHomeDirectory /Users/Guest
		#setting up an empty password and giving local Kerberos some time to process it
		${DSCL} . -passwd /Users/myguest ''
		sleep 2
		${DSCL} . -create /Users/myguest Picture "/Library/User Pictures/Nature/Leaf.tif"
		${DSCL} . -create /Users/myguest PrimaryGroupID 201
		${DSCL} . -create /Users/myguest RealName "Local Guest"
		${DSCL} . -create /Users/myguest RecordName myguest
		#OS X does not like two users with same UUID so don't use 201 on the next line
		${DSCL} . -create /Users/myguest UniqueID 401
		${DSCL} . -create /Users/myguest UserShell /bin/bash
		#Adding the keychain item that allows Guest to login in 10.8
		${SECURITY} add-generic-password -a myguest -s com.apple.loginwindow.guest-account -D "application password" /Library/Keychains/System.keychain
		;;
	13)
	#OS X Maveriks	
		${LOGGER} -s -t createmyguest.sh "INFO: executing the 10.9 version of the script"
		${DSCL} . -create /Users/myguest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_defaultLanguage en
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_guest true
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers__defaultLanguage Guest
		${DSCL} . -create /Users/myguest dsAttrTypeNative:_writers_UserCertificate Guest
		${DSCL} . -create /Users/myguest AuthenticationHint ''
		${DSCL} . -create /Users/myguest NFSHomeDirectory /Users/Guest
		#setting up an empty password and giving local Kerberos some time to process it
		${DSCL} . -passwd /Users/myguest ''
		sleep 2
		${DSCL} . -create /Users/myguest Picture "/Library/User Pictures/Nature/Leaf.tif"
		${DSCL} . -create /Users/myguest PrimaryGroupID 201
		${DSCL} . -create /Users/myguest RealName "Local Guest"
		${DSCL} . -create /Users/myguest RecordName myguest
		#OS X does not like two users with same UUID so don't use 201 on the next line
		${DSCL} . -create /Users/myguest UniqueID 401
		${DSCL} . -create /Users/myguest UserShell /bin/bash
		#Adding the keychain item that allows Guest to login in 10.9
		${SECURITY} add-generic-password -a myguest -s com.apple.loginwindow.guest-account -D "application password" /Library/Keychains/System.keychain
		;;
	*)
	#Any other version
		${LOGGER} -s -t createmyguest.sh "ERROR: Operating system version not supported by this script"
		exit -1;;
esac

${LOGGER} -s -t createmyguest.sh "INFO: Exiting"

exit 0