In addition to the default user that you specified when you set up the database, you can create additional database users. Simply enter the login name and password under "Add a new MS SQL Login." 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.

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.
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