Example of pulling the primary registrant object from within the event based shortcodes.

example.php

<?php

function register_new_tony_shortcodes( $shortcodes, EE_Shortcodes $lib ) {

    //Add a shortcode to be used with the EE Event List within messages
    if ( $lib instanceof EE_Event_Shortcodes ) {
        //Add your shortcode to the add as the key, the value should be a description of the shortcode.
        $shortcodes['[EVENT_URL_GENERATE]'] = _('The shortcode to generate the access url');
    }
    //Return the shortcodes.
    return $shortcodes;
}
add_filter( 'FHEE__EE_Shortcodes__shortcodes', 'register_new_tony_shortcodes', 10, 2 );
function register_new_tony_shortcodes_parser( $parsed, $shortcode, $data, $extra_data, EE_Shortcodes $lib ) {
   
    if ( $lib instanceof EE_Event_Shortcodes )
    {
        //Then check for our specific shortcode
        if ( $shortcode == '[EVENT_URL_GENERATE]' )
        {
            //global $wpdb;
            //First check we have an event object.
            //You don't need the EE_Event object for this functionality, so this isn't needed currently.
            $event = $data instanceof EE_Event ? $data : null;
            
            //Pull the EE_Messages_Addressee object from extra data if it is available.
            $aee = is_array( $extra_data ) && !empty( $extra_data['data'] ) && $extra_data['data'] instanceof EE_Messages_Addressee ? $extra_data['data'] : null;

            if ($aee ! instanceof EE_Messages_Addressee ) {
              //After debugging I'd advise you change this to return an empty string.
              return 'Object not Found';
            }

            $primary_reg = $aee->primary_reg_obj;
            $primary_attendee = $aee->primary_att_obj;
           
            if( $primary_reg instanceof EE_Registration )
            {
                //Is a valid EE_Registration object, do something with it here.
 
            }
           
            if ( $primary_attendee instanceof EE_Attendee )
            {
                //$primary_attendee is a valid EE_Attendee object, as it's the primary
                //attendee object you can pull the email from it.
                $primary_att_email = $primary_attendee->email();
                return 'Result ' . $primary_att_email;
            }
        }
    }
    //If not within the correct section, or parsing the correct shortcode,
    //Return the currently parsed content.
    return $parsed;
}
add_filter( 'FHEE__EE_Shortcodes__parser_after', 'register_new_tony_shortcodes_parser', 10, 5 );