You can use the script step 'Add Account' to let them do that. You could show the user a Custom Dialogue that lets them specify the User name and Password they want. You can use the 3rd entry field to confirm the password.
You need to also specify the Privilege set they will be a member of. You can lock that into your script (by only allowing them to create users at one level) or you can offer another dialogue to ask them what level of access they want.
You would probably want to restrict that script to users of a certain level, of course.