See this page for details and usage instructions: http://www.atrixnet.com/send-an-email-when-someone-logs-in/

iloggedin.sh

#!/bin/bash
RECIPIENT="your.email@address.here";
PREFIX="LOGIN ALERT!";
REMOTEIP=$(/bin/echo $SSH_CLIENT | /usr/bin/awk '{ print $1 }');
TIME=$(/bin/date +'%r, %D');
HOST=$(/bin/hostname -f);
 
if [[ "$REMOTEIP" == "" ]]; then
   REMOTEIP='localhost';
fi
 
/bin/cat <<LOGGEDIN | /usr/bin/mail -s "$PREFIX $USER@$HOST $TIME" $RECIPIENT
Remote user $USER just logged in to $HOST at $TIME from $REMOTEIP
LOGGEDIN
 
exit $?

nice