Multisite: Share users and user roles for existing set up
You don't need additional modules to achieve what you want.

First: Fix you configuration:

'prefix' =&gt; array(
'default'   =&gt; 'default_db.',
'users'     =&gt; 'myhost_users.', //
'sessions'  =&gt; 'myhost_users.',
'role'      =&gt; 'myhost_users.',
'authmap'   =&gt; 'myhost_users.',
),


The dots on the end of 'users' =&gt; 'myhost_users.' tells Drupal to query the users table in myhost_users data base (SQL syntax); without dots you'll get an error like "Base table or view not found myhost_usersusers/myhost_users_users or similar.

Now,

'database' =&gt; 'db',
'username' =&gt; 'db_user',
'password' =&gt; 'db_user_pass',


are credentials for the default db where Drupal is installed. To query users db you need to add this user with his password (db_user, db_user_pass) to the myhost_users database. in other words, if you have a site with 2 subsites:


myhost_site1 - user of this db
myhost_site2 - user of this db
myhost_users - (assuming this db stores Drupal users, that you want to share across ites) user of this sb + user of myhost_site1 and user of myhost_site2


To stay logged in on each site it is necessary to share cookies between sites: edit settings.php file of each site, uncomment the line with # $cookie_domain = '.example.com'; and replace '.example.com' with your domain. Don't forget the dot at the beginning ('.mysite.com') -- without it both mysite.com and notmysite.com could be treated as the same domain.