In StresStimulus you can emulate multiple VUs with different credentials. The required configuration will depend on the type of authentication your application uses.
- If your application uses Basic authentication, then in the Authentication area specify the correct usernames and passwords for your virtual users. You can import users' credentials from a .csv file with the same 3 columns as in the grid by clicking the "Import" button.

- If your application uses Windows integrated authentication (NTLM / Kerberos), then use the same approach as above, but also populate the Domain column.
- If your application uses the form-based authentication, where credentials are sent via post request, use the Parameterization section. To do so:
Record a test case that includes log-in page navigation.
Create a .csv file, where each record contains a valid user name and a password. If store the credentials in Excel file, save it as .csv.
Go to the Test Case-> Parameterization -> Data Sources, and add the .csv file. You can change the Data Source name to a meaning full name, let say "TestUsers"

- Click on the arrow down on the “Show … “ split button and select “show test case request - Primary”. After that select the log-in page in the Fiddler grid.
- Go to the Test Case-> Parameterization -> Requests and databind the user name and the password fields with the .csv file columns. ; select VU-bound data-binding method.

To read more about the data-binding methods, check this blog, item #3.
To read more about parameterization check this section of the paper at about advanced StresStimulus features: http://www.codeproject.com/KB/applications/stresstimulus2.aspx