Validation error, controller and path


I have a controller and model called user. Then I have a controller
called signup and there I have a form for adding a user which is going
to an action in user. How do I make sure that I get back to that same
controller/view when there is a validation error?

Either that or having the form in the controller and view user but make
sure that the path is /signup and not /users/signup. How do I do that?