Because this is a recurring question, I've created a help page about it: https://www.mediawiki.org/wiki/Help:Mass_account_creation I hope it helps. It's already quite verbose, so I've kept it generic enough and avoided all edge cases and solutions. This is really not a problem, you just have to know about it; in my chapter no problem has ever occured with it in the last 4-5 years (ok, it's more difficult if you're not a sysop, but only a bit). ;-)
Nemo