Try setting the notification to true.

sendNotifications

• Whether to send notifications about the creation of the new event. Optional. The default is False.

In the Try this API (Events: insert):

I'm not very familiar with java but try using this code as suggested in a related SO post.

Event createdEvent = calendario.events().insert("some-mail-from-google0@group.calendar.google.com", event).setSendNotifications(true).execute();

Hope this helps.

sendNotifications is depreciated.

You should use "sendUpdates" in de querystring, e.g. javascript:

    let request = gapi.client.calendar.events.insert({
        "calendarId": workCalendar,
        "resource": newEvent,
        "sendUpdates": "externalOnly"
    });

you need to add .setSendNotifications(true) just before execute(). this line is not there in google calendar example