is there anything that will automatically create users for my MyBB forum?
Define 'automatically'... you can create users in the ACP, but I'm not sure what you mean by 'automatically'.
Do you want fake users? Do you want full accounts with sigs and avatars? Do you want to just import a large quantity of users? Do you want fake users with posts?
In admincp you can manually add users to the system. I am sure a plugin could be made to create dozens of forum users rather easily. The question is do you really want that?
I think he wants a plugin that create automaticcaly some users!
I'm quite sure that you could create a simple mysql query in a new php file and add it as a task.