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 New MS SQL 2000 User."
You may also change the password of your default user (or any additional users) by clicking the "change password" link associated with the user you wish to change.
It's important to note that adding a user here is only the first step. Read this Knowledge Base article for details on giving your newly created users the proper permissions to access to your database.