Yes, we regularly launch up to 1,000 browser users -- each one acting like an independent and separate user...
There's not trick really. Each user account is set up to launch an eValid load test script at a specific time...the scripts all use the WaitMod commands in eValid to delay the start of each script until a specific time.
So if you want to have your whole run-up to 1,000 users start at say time T1, then account #1 does a wait until T1, account #2 waits until time T1+100, #3 until T1+200, and so forth.
What we do is get all 10 user accounts all set up and waiting about 10 minutes before trigger time T1. Afte that it is all on autopilot. We just site back, watch the load grow... and watch how the server fails out...
Here's the manual page that describes these special commands: WaitMod Processing Explanation