A bug maybe; The friend inviter page welcomes you with an error if you haven't set your Yahoo api:
#0 /home/user/public_html/library/FriendInviter/Listener.php(121): XenForo_Application::handlePhpError(8, 'Undefined varia...', '/home/divnet/pu...', 121, Array)
#1 [internal function]: FriendInviter_Listener::addNavbarTab(Array, 'friend-inviter')
#2 /home/user/public_html/library/XenForo/CodeEvent.php(54): call_user_func_array(Array, Array)
#3 /home/user/public_html/library/XenForo/Dependencies/Public.php(451): XenForo_CodeEvent::fire('navigation_tabs', Array)
#4 /home/user/public_html/library/XenForo/Dependencies/Public.php(335): XenForo_Dependencies_Public->_getNavigationContainerParams('friend-inviter')
#5 /home/user/public_html/library/XenForo/FrontController.php(563): XenForo_Dependencies_Public->getEffectiveContainerParams(Array, Object(Zend_Controller_Request_Http))
#6 /home/divnet/public_html/library/XenForo/FrontController.php(156): XenForo_FrontController->renderView(Object(XenForo_ControllerResponse_View), Object(XenForo_ViewRenderer_HtmlPublic), Array)
#7 /home/user/public_html/index.php(13): XenForo_FrontController->run()
#8 {main}
ErrorException: Undefined variable: yahooEnabled - library/FriendInviter/Listener.php:121
There isn't an option to turn it off in the settings page. I couldn't get the Yahoo inviter to work after I set a Yahoo app up. Getting this error:
There has been a problem communicating with Yahoo.