Added missing translation message

locale/en_US/messages.php

@@ -61,6 +61,7 @@ return [
 
             'EXPLAIN'        => 'Please enter a name for the organisation',
             'IN_USE'         => 'Organisation name <strong>{{name}}</strong> is already in use.',
+            'IN_USE_REGISTER'           => 'Organisation name <strong>{{name}}</strong> is already in use. Perhaps you would like to request to join it instead?',
         ],
         'SLUG' => [
             'IN_USE'         => 'Organisation slug <strong>{{slug}}</strong> is already in use.',