How do I create a multisite with  shared users and nodes?
You can change the settings for the access to the database that are contained in the settings.php file used by your Drupal site to something similar to the following one (replace the values between brackets with the correct values for your site):

$databases['default']['default'] = array(
  'driver' =&gt; '[Your database engine]',
  'database' =&gt; '[Your database name]',
  'username' =&gt; '[Your username for the access to the database engine]',
  'password' =&gt; '[The password for the access to the database]',
  'host' =&gt; 'localhost',
  'prefix' =&gt; array(
    'default'   =&gt; 'main_',
    'users'     =&gt; 'shared_',
    'sessions'  =&gt; 'shared_',
    'role'      =&gt; 'shared_',
    'authmap'   =&gt; 'shared_',
  ),
  'collation' =&gt; 'utf8_general_ci',
);