The username merge field is only available in Email Messages that have a Merge Data Source of "Registered Users". For security purposes, the passwords are encrypted in the database and cannot be extracted to be sent in an email message. We recommend sending a link to the user login page on your site and instruct users to follow the forgotten password link to reset their password.

To send an email to a select group of individuals, an imported list with columns for username and password must be used.  
For more information on creating and sending emails, please see the Email Guide.