Load Tests - parallel login of users in J2EE application.

I am new to soapUI/loadUI and I am trying to do load-tests for some J2EE based applications. For, my tests I need to login large number of registered users (around 2000 users in parallel) then each user does a particular transaction on the site and then logouts of the application. Each of the above steps (login - transaction - logout) happens as HTTP requests (POST). There is a constraint that when a a particular user is logged in on to the application he cannot login again till he logs out the original login.

How can i do this test using SoapUI/loadUI? The main problem in using soapUI/loadUI for these tests is that I do not know how to login 2000 different users (with all different user names) in parallel, given that login happens as a HTTP request to a url (POST). Upto now, in soapUI, I have managed to create a test case, where, I login a single user (1st HTTP request step), do the transaction(2nd HTTP step) and logout (3rd HTTP request step). How do i now do load tests of this test case, ie, how do i generalise the user name parameter (in the first HTTP request step) so athat the load test logs in all 2000 users in parallel?

Thanks very much.