Multi-site installation with shared database but different user table
You can try editing you sub-sites settings.php file and you just need to provide different prefix for user table (as you need only this table different).

I am providing sample here so that you can implement and see if it works.  

$databases = array (
  'default' =&gt; 
  array (
    'default' =&gt; 
    array (
      'database' =&gt; 'drupal',
      'username' =&gt; 'root',
      'password' =&gt; '',
      'host' =&gt; 'localhost',
      'port' =&gt; '',
      'driver' =&gt; 'mysql',
      'prefix' =&gt; array(
           'users'     =&gt; 'abcsite_',
      ),
    ),
  ),
);


The above code will tell your installation to use abcsite_users (you need to create table first with same structure as of default users table) for entering user details.