wpmem_pre_validate_form

add_filter( 'wpmem_pre_validate_form', 'my_pre_validation_filter', 10, 2 );
 
function my_pre_validation_filter( $fields, $tag ) {
    /**
     * The data from the registration form is brought in 
     * with the $fields array.  You can filter any of the
     * the values, and add/subtract from the array before
     * returning the filtered result.
     */
 
    return $fields;
}