diff --git a/src/FormRequest/CreateUserRequest.php b/src/FormRequest/CreateUserRequest.php
index 5e0b740..523429c 100644
--- a/src/FormRequest/CreateUserRequest.php
+++ b/src/FormRequest/CreateUserRequest.php
@@ -3,6 +3,7 @@
 namespace App\FormRequest;
 
 use Symfony\Component\Validator\Constraints as Assert;
+use App\Validator\Constraints as AppAssert;
 
 /**
  * @todo implement UniqueEntity constraint for DTO and use it here
@@ -37,6 +38,7 @@ class CreateUserRequest
      *
      * @Assert\NotBlank()
      * @Assert\Length(min="32", max="32")
+     * @AppAssert\ValidInvite()
      */
     public $inviteCode;
 
diff --git a/src/Validator/Constraints/ValidInvite.php b/src/Validator/Constraints/ValidInvite.php
new file mode 100644
index 0000000..1a0d455
--- /dev/null
+++ b/src/Validator/Constraints/ValidInvite.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace App\Validator\Constraints;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * @Annotation
+ */
+class ValidInvite extends Constraint
+{
+    public $notFoundMessage = 'Invite {{ code }} not found.';
+    public $usedMessage = 'Invite {{ code }} is used.';
+
+    public function validatedBy(): string
+    {
+        return get_class($this).'Validator';
+    }
+}
\ No newline at end of file
diff --git a/src/Validator/Constraints/ValidInviteValidator.php b/src/Validator/Constraints/ValidInviteValidator.php
new file mode 100644
index 0000000..b192b69
--- /dev/null
+++ b/src/Validator/Constraints/ValidInviteValidator.php
@@ -0,0 +1,42 @@
+<?php
+
+namespace App\Validator\Constraints;
+
+use App\Entity\Invite;
+use App\Repository\InviteRepository;
+use Symfony\Component\Validator\{Constraint, ConstraintValidator};
+
+class ValidInviteValidator extends ConstraintValidator
+{
+    /** @var InviteRepository */
+    private $inviteRepo;
+
+    public function __construct(InviteRepository $inviteRepo)
+    {
+        $this->inviteRepo = $inviteRepo;
+    }
+
+    /**
+     * @param mixed $value
+     * @param ValidInvite $constraint
+     */
+    public function validate($value, Constraint $constraint)
+    {
+        /** @var Invite $invite */
+        if (null === $invite = $this->inviteRepo->findOneBy(['code' => $value])) {
+            $this->context->buildViolation($constraint->notFoundMessage)
+                ->setParameter('{{ code }}', $value)
+                ->addViolation()
+            ;
+
+            return;
+        }
+
+        if (null !== $invite->getUsedBy()) {
+            $this->context->buildViolation($constraint->usedMessage)
+                ->setParameter('{{ code }}', $value)
+                ->addViolation()
+            ;
+        }
+    }
+}
\ No newline at end of file