You can create this by following this simple steps
1> create one folder named common inside your document root
2> now place drupal folder there which you have donwloaded
3> now rename it with any other name
4> now go to sites and remove all directory
5> now create folder with name of domains of your site names
6> now crate your cutom folders named "modules" and "themes" there as you was first creating inside sites directory
7> now copy settings.php from default folder and copy them in each domain directories we have created
8> now edit them for database name and username and password of that
do following changes on given line in each settings.php
line number :93
$db_url = 'mysql://usernamel:password@localhost/databasename';
now give prefix to some tables which are as following... you can give prefix to more tables to if u keep them different for all site
$db_prefix = array(
'default' =>'',
'menu' => 'first_',
'variable' => 'first_',
'cache' => 'first_',
'cache_menu'=> 'first_',
);
now after making this ... go to browser and type like this "domain of your first site/install.php"
now it will show page that your database for site is created and you can visit new site by a link shown there.
now do one thing befor you go and install second site.... you go to database and delete all othere tables that dont have prefix....
then install second site... and repeate above steps each time you create a new site....
but dont delete it after you create last site.....
now your sites are ready with common users and content....enjoy!
and yea dont forget to make relative path changes in http.conf if this thing concerns you....
and now enjoy making multi site with same core modules .....!