diff --git a/app/Http/ValidationRules/User/AttachableUser.php b/app/Http/ValidationRules/User/AttachableUser.php
new file mode 100644
index 000000000000..6c572cc32f78
--- /dev/null
+++ b/app/Http/ValidationRules/User/AttachableUser.php
@@ -0,0 +1,73 @@
+<?php
+/**
+ * Invoice Ninja (https://invoiceninja.com).
+ *
+ * @link https://github.com/invoiceninja/invoiceninja source repository
+ *
+ * @copyright Copyright (c) 2021. Invoice Ninja LLC (https://invoiceninja.com)
+ *
+ * @license https://opensource.org/licenses/AAL
+ */
+
+namespace App\Http\ValidationRules\User;
+
+use App\Models\CompanyUser;
+use App\Models\User;
+use Illuminate\Contracts\Validation\Rule;
+
+/**
+ * Class AttachableUser.
+ */
+class AttachableUser implements Rule
+{
+
+    public function __construct()
+    {
+    }
+    /**
+     * @param string $attribute
+     * @param mixed $value
+     * @return bool
+     */
+    public function passes($attribute, $value)
+    {
+        return $this->checkUserIsAttachable($value);
+    }
+
+    /**
+     * @return string
+     */
+    public function message()
+    {
+        return "Cannot add the same user to the same company";
+    }
+
+    /**
+     * @param $user_id
+     * @return bool
+     */
+    private function checkUserIsAttachable($email) : bool
+    {
+        if (empty($email)) {
+            return false;
+        }
+
+        $user = User::where('email', $email)->first();
+
+        if(!$user)
+            return true;
+
+        $user_already_attached =  CompanyUser::query()
+                                    ->where('user_id', $user->id)
+                                    ->where('account_id',$user->account_id)
+                                    ->where('company_id', auth()->user()->company()->id)
+                                    ->withTrashed()
+                                    ->exists();
+
+        if($user_already_attached)
+            return false;
+
+
+        return true;
+    }
+}