Example of how to hide the Personal information question group's email question using CSS if the current user is logged in.

tw_ee_hide_primary_email.php

<?php //Please do not include the opneing PHP tag if you already have one.
function tw_ee_hide_primary_email(){
    if( is_user_logged_in() ) {
        wp_add_inline_style( 
            'single_page_checkout',
            'div[id*="ee-reg-form-qstn-grp-personal-information"] .ee-reg-qstn-email {
                display: none;
            }'
        );
    }
}
add_action( 'wp_enqueue_scripts', 'tw_ee_hide_primary_email', 50 );