Much work.

app/controllers/registrations_controller.rb

@@ -0,0 +1,31 @@
+class RegistrationsController < ApplicationController
+  allow_unauthenticated_access only: [:new, :create]
+  before_action :ensure_no_users_exist, only: [:new, :create]
+
+  def new
+    @user = User.new
+  end
+
+  def create
+    @user = User.new(user_params)
+
+    if @user.save
+      start_new_session_for(@user)
+      redirect_to root_path, notice: "Welcome! Your admin account has been created successfully."
+    else
+      render :new, status: :unprocessable_entity
+    end
+  end
+
+  private
+
+  def user_params
+    params.require(:user).permit(:email_address, :password, :password_confirmation)
+  end
+
+  def ensure_no_users_exist
+    if User.exists?
+      redirect_to new_session_path, alert: "Registration is not allowed. Users already exist."
+    end
+  end
+end