UPDATE: A few of you have (very fairly) asked why we aren't supporting the feature anymore. The reason is basically because of low usage: amongst our Gmail users, this feature wasn't getting as much usage as others do. Again, we'll still of course support invitation creation, but it's in a different spot now (from Google Calendar).
Thanks for the good question.
If you're trying to use the "insert invitation" feature, you're likely now seeing a message that reads: "Starting July 2014, this feature will no longer be available. You can continue sending email invitations from Google Calendar."
It (truly!) is always tough choosing which features to continue to support. We make these decisions carefully, so please know that! We'll still be supporting creating invitations, just through different steps. The workaround now is to click into calendar and to create an invitation from there. See this article on creating an invitation in Calendar for more info.