In addition to the default user created when you set up the database, you can create three additional database users. Simply enter the login name and password under "Add a new MS SQL 2005 Login."
You may also change the password of your default user (or any additional users) by clicking the "change password" link for the user you wish to change.
It's important to note that adding a user here is only the first step. Read this article for details on giving your newly created users the proper permissions to access to your database.