<%= form_for @user1 do |f| %>

    Username: <%= f.text_field :username %>      <br />
    Password: <%= f.password_field :password %>  <br />.
    Email:    <%= f.email_field :email %> <br />

    <%= submit_tag "Add" %>

<% end %>

undefined method `users_path' for #<#<Class:0x000000020576f0>:0x0000000335d5d8>

class UserController < ApplicationController
  def add
    @user1 = User.new
    respond_to do |format|
      format.html
      format.json {render :json => @user1}
    end
  end

Rails.application.routes.draw do
  resources :users # or resource :user for singular resources
  ...
end

resources :user

resources :users