Drush not working on Multisite with Shared User Table
The solution ended up being very simple.

I just had to change one line.

I changed this:

        'default'   =&gt; '[Multisite DB].',


to:

        'default'   =&gt; '',


This is how it looks all together.

$databases['default']['default'] = array(
  'driver' =&gt; 'mysql',
  'database' =&gt; '[Multisite DB]',
  'username' =&gt; '[Database Username]',
  'password' =&gt; '[Database Password]',
  'host' =&gt; '[DB Server IP]',
  'charset' =&gt; 'utf8mb4',
  'collation' =&gt; 'utf8mb4_general_ci',
  'prefix' =&gt; array(
        'default'   =&gt; '',
        'users'     =&gt; '[Main Site DB].',
        'sessions'  =&gt; '[Main Site DB].',
        'role'      =&gt; '[Main Site DB].',
        'authmap'   =&gt; '[Main Site DB].',
        'users_roles'   =&gt; '[Main Site DB].',
        'field_data_field_first_name'   =&gt; '[Main Site DB].',
        'field_data_field_last_name'   =&gt; '[Main Site DB].',
        'field_data_field_title'   =&gt; '[Main Site DB].',
  ),
);


I know that a lot of people use Bakery or are already on Drupal 8, but I hope this helps anyone still on Drupal 7 looking to share a User table across multiple websites with different databases.