There is only one account, so just check join code directly

app/controllers/users_controller.rb

@@ -2,7 +2,7 @@ class UsersController < ApplicationController
   require_unauthenticated_access only: %i[ new create ]
 
   before_action :set_user, only: %i[ show edit update destroy ]
-  before_action :set_account_from_join_code, only: %i[ new create ]
+  before_action :ensure_join_code_is_valid, only: %i[ new create ]
   before_action :ensure_permission_to_administer_user, only:  %i[ update destroy ]
 
   def index
@@ -36,8 +36,8 @@ class UsersController < ApplicationController
   end
 
   private
-    def set_account_from_join_code
-      @account = Account.find_by_join_code!(params[:join_code])
+    def ensure_join_code_is_valid
+      head :forbidden unless Account.sole.join_code == params[:join_code]
     end
 
     def set_user

app/views/users/new.html.erb

@@ -11,7 +11,7 @@
 <div class="panel shadow center">
   <h1 class="margin-none-block-start margin-block-end-double">Fizzy</h1>
 
-  <%= form_with model: @user, url: join_path(@account.join_code), class: "flex flex-column gap" do |form| %>
+  <%= form_with model: @user, url: join_path(params[:join_code]), class: "flex flex-column gap" do |form| %>
     <div class="flex align-center gap">
       <label class="flex align-center gap input input--actor">
         <%= form.text_field :name, class: "input full-width", autocomplete: "name", placeholder: "Name", autofocus: true, required: true, data: { "1p-ignore": true } %>