feat: Add accept invitation endpoint (#343)

Co-authored-by: Garen J. Torikian <gjtorikian@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

lib/UserManagement.php

@@ -643,6 +643,29 @@ public function resendInvitation($invitationId)
         return Resource\Invitation::constructFromResponse($response);
     }
 
+    /**
+     * Accept an Invitation
+     *
+     * @param  string  $invitationId  ID of the Invitation
+     * @return Resource\Invitation
+     *
+     * @throws Exception\WorkOSException
+     */
+    public function acceptInvitation($invitationId)
+    {
+        $path = "user_management/invitations/{$invitationId}/accept";
+
+        $response = Client::request(
+            Client::METHOD_POST,
+            $path,
+            null,
+            null,
+            true
+        );
+
+        return Resource\Invitation::constructFromResponse($response);
+    }
+
     /**
      * Generates an OAuth 2.0 authorization URL used to initiate the SSO flow with WorkOS.
      *

tests/WorkOS/UserManagementTest.php

@@ -1678,6 +1678,50 @@ public function testResendInvitationAccepted()
         $this->userManagement->resendInvitation($invitationId);
     }
 
+    public function testAcceptInvitation()
+    {
+        $invitationId = "invitation_01E4ZCR3C56J083X43JQXF3JK5";
+        $path = "user_management/invitations/{$invitationId}/accept";
+
+        $result = $this->invitationResponseFixture();
+
+        $this->mockRequest(
+            Client::METHOD_POST,
+            $path,
+            null,
+            null,
+            true,
+            $result
+        );
+
+        $response = $this->userManagement->acceptInvitation($invitationId);
+
+        $expected = $this->invitationFixture();
+
+        $this->assertSame($response->toArray(), $expected);
+    }
+
+    public function testAcceptInvitation404()
+    {
+        $invitationId = "invitation_01E4ZCR3C56J083X43JQXF3JK5";
+        $path = "user_management/invitations/{$invitationId}/accept";
+
+        $this->expectException(Exception\NotFoundException::class);
+
+        $this->mockRequest(
+            Client::METHOD_POST,
+            $path,
+            null,
+            null,
+            true,
+            null,
+            null,
+            404
+        );
+
+        $this->userManagement->acceptInvitation($invitationId);
+    }
+
     public function testGetJwksUrl()
     {
         $clientId = "12345";