It is fairly easy to use one Drupal 7 code base for several sites. However, you need one database for each site, so you cannot share users across the sites. If you know an easy workaround to share users, please advice in the comments.
What I did?
I did the installation on a Dreamhost shared server with ssh access.
First, I created one database for each site.
Second, I created the domains. When you create a domain at Dreamhost, it will create a web accessible folder:
