I added it to the rsvp_frontend.inc.php script;
On line 739 you will find this;
($sql = "SELECT firstName, lastName, rsvpStatus FROM ".ATTENDEES_TABLE." WHERE id= ".$attendeeID;)
Under this line I added 2 lines;
$ip = $_SERVER['REMOTE_ADDR'];
$browser = $_SERVER['HTTP_USER_AGENT'];
And under line 745 I added this;
$body .= "$ip | $browser";
This will send the IP-address and browser info in the notify e-mail.