How to share user and user profile in a multisite environment?
Edit settings.php -> $databases

$databases['default']['default'] = array(
  'database' =&gt; 'my_db',
  'username' =&gt; 'user',
  'password' =&gt; 'pass',
  'host' =&gt; 'example',
  'prefix' =&gt; array(
    'default'   =&gt; '',
    'users'     =&gt; 'shared_db_',
    'sessions'  =&gt; 'shared_db_',
    'role'      =&gt; 'shared_db_',
    'authmap'   =&gt; 'shared_db_',
  ),
  'collation' =&gt; 'utf8_general_ci',
);


Add any other tables as required in prefix array.
Have a look at settings.php as well it has lots of documentation on all these.