What happened to the JUnit plugin?
The Grinder 2 had a JUnit plugin which allowed JUnit test cases to be called. There is no JUnit plugin in The Grinder 3 but you can call arbitrary Java code so you should be able to invoke your JUnit test cases directly. If you have a lot of JUnit test cases, it would be appropriate to wrap up the steps necessary to invoke a test case in a Jython library. Contributions to the script gallery are welcome. See also this article on grinder-use.