If u want to create an event using built-in Calendar app for iPhone, iPod touch, and iPad, and u want to invite ur friends and alert them about it. There are few simple steps..

1. How to create an event and share it with ur friends..

Open Calendar app
Either create a new event or tap an existing event u’d like to share.
Tap the Edit button (in the upper right hand corner).
Tap on Invitees button (under the Edit options menu).
Either start typing the name of a contact, or tap on the blue + button.
Tap the contact name and, if they have multiple email addresses, tap the email address. U can add multiple people using above method.
Tap the Done button in the upper right hand corner when u’re finished. And The invitation will be sent by email to everyone u’ve added.

2. How to accept an event invitation sent by any of ur friend using apple device.

Open Calendar app.
Tap on the Inbox button in the lower right hand corner, or tap on the grayed out event that appears outlined in ur event list.
Tap accept to add the event to ur calendar.
U can Tap decline or maybe to refuse the invitation or wait so u can decide later.