Example of how to hide the 'Personal Information' email question for all additional registrants and then copy the email address set by the Primary Registrant to all other email fields.

all-events.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_ee_copy_primary_email_to_all() {
	wp_add_inline_style( 
		'single_page_checkout',
		'.spco-attendee-panel-dv:not(:first-of-type) .ee-reg-qstn-email-input-dv, .spco-attendee-panel-dv:not(:first-of-type) .ee-reg-qstn-email {
			display: none;
		}'
	);
	
	wp_add_inline_script( 
		'single_page_checkout',
		'jQuery("input.ee-reg-qstn-email").change(function() {
			jQuery("input.ee-reg-qstn-email").val(jQuery(this).val());
		});'
	);
}
add_action( 'wp_enqueue_scripts', 'tw_ee_copy_primary_email_to_all', 11 );

custom-value-events.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_ee_copy_primary_email_to_all() {
    $checkout = EE_Registry::instance()->SSN->checkout();
    if ( $checkout instanceof EE_Checkout ) {
        $transaction = $checkout->transaction;
        if ( $transaction instanceof EE_Transaction ) {
            foreach ( $transaction->registrations() as $registration ) {
                if ( $registration instanceof EE_Registration ) {
                    $event = $registration->event();
                    if( $event->get_post_meta('hide_email_fields', true ) ) {
                        wp_add_inline_style( 
                            'single_page_checkout',
                            '.spco-attendee-panel-dv:not(:first-of-type) .ee-reg-qstn-email-input-dv, .spco-attendee-panel-dv:not(:first-of-type) .ee-reg-qstn-email {
                                display: none;
                            }'
                        );
                        
                        wp_add_inline_script( 
                            'single_page_checkout',
                            'jQuery("input.ee-reg-qstn-email").change(function() {
                                jQuery("input.ee-reg-qstn-email").val(jQuery(this).val());
                            });'
                        );
                    	break;
                    }
                }
            }
        }
    }
}
add_action( 'wp_enqueue_scripts', 'tw_ee_copy_primary_email_to_all', 11 );