Use the Restrict Content plugin to restrict Event Espresso event content. You can add this code to a site specific plugin.

restrict_content_ee.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function my_ee_rcAddMetaBoxes() {
  if(function_exists('rc_get_metabox') && post_type_exists('espresso_events')){
    $metabox = rc_get_metabox();
    $post_type = get_post_type_object('espresso_events');
    add_meta_box( 
      $metabox['id'], 
      $metabox['title'], 
      'rcShowMetaBox', 
      $post_type->name, 
      $metabox['context'], 
      $metabox['priority'] 
    );
  }
}
add_action('admin_menu', 'my_ee_rcAddMetaBoxes', 11);
function my_ee_rcCheckUser($content) {
  if (
    function_exists('rcCheckUser') && 
    'espresso_events' == get_post_type()
    ) {
    global $post;
    if (
      get_post_meta($post->ID, 'rcUserLevel', true) &&
      has_filter('the_content', 'rcMetaDisplayNone') 
    ) {
      remove_filter(
        'the_content', 
        array('EED_Event_Single', 'event_details'),
        EED_Event_Single::EVENT_DETAILS_PRIORITY
      );
      remove_filter(
        'the_content', 
        array('EED_Events_Archive', 'event_details'),
        EED_Events_Archive::EVENT_DETAILS_PRIORITY
      );
      add_filter(
        'the_content',
        function(){
          global $post;
          $html = $post->post_content;
          $html .= '<br>You need to <a href="' . wp_login_url( get_permalink() ) . '">log in</a> to register for this event. ';
          $html .= 'You can <a href="' . wp_registration_url() . '">sign up for an account here</a>.';
          echo $html;
        },
        999
      );
    }
  }
  return($content);
}
add_filter('the_content', 'my_ee_rcCheckUser', 11);