If you are looking for a quick way to create test users in your domain, you can do so through the old NET USER command. You will have to run this on your domain controller’s command prompt. The following will create 200k test users:
for /l %i in (1,1,200000) do net user TestUser%i p@ssw0rd /domain /add
If you need to tweak more of the details of the test accounts, there are a number of other options for the NET USER command, but you most likely will want to take a look at the more robust tools out there like csvde.
Thanks to Daniel Petri for this little nugget. For a more in depth view of your options for quickly creating test users, check out his post here:
Hope this helps!