I have done something similar using a batch file that asks for input and uploads a file.auto.rsc script via ftp to the router. Upon upload, the script is automatically executed. Downside is that router password would be in plain text in the batch file, and with some versions of RouterOS, there is a bug I ran into.
A second option would be creating a program in java/php/C/whatever using the API. Using this method you would have to turn on the API service on each router.
Another option would be to create a custom Webfig skin. They would have to log in via their browser to add the users.