JMeter is perfect for the job! You will need a CSV file with 300 names and passwords. Then you would need to use CSVDataSetConfig: http://jmeter.apache.org/usermanual/component_reference.html#CSV_Data_Set_Config Here is a link to a short tutorial: http://blazemeter.com/blog/jmeter-tutorial-video-series Video #4 and #5 describe exactly what you need.
BTW, same functionality is supported by blazemeter, free of charge up to a level of 50 concurrent users.