new_contents: from django.contrib import admin
  
  from django.contrib.auth.admin import UserAdmin
  from django.contrib.auth.forms import UserCreationForm
  from django.forms import forms
  from guardian.admin import GuardedModelAdmin
  from models import CustomUser
  
  
  class CustomUserCreationForm(UserCreationForm):
      def clean_username(self):
          # Since User.username is unique, this check is redundant,
          # but it sets a nicer error message than the ORM. See #13147.
          username = self.cleaned_data["username"]
          try:
              CustomUser._default_manager.get(username=username)
          except CustomUser.DoesNotExist:
              return username
          raise forms.ValidationError(self.error_messages['duplicate_username'])
  
      class Meta(UserCreationForm.Meta):
          model = CustomUser
  
  
  class CustomUserAdmin(GuardedModelAdmin, UserAdmin):
      add_form = CustomUserCreationForm
  
  
  admin.site.register(CustomUser, CustomUserAdmin)
  
subject: add admin page for users