When setting up your email, there will be two places you can add a calendar invite. The first is during the design step. Check out the Advanced Options section below the right-side Layout panel. Find the Attach Calendar Invite (.ICS file) option. Ticking this box will automatically pull the details of your event--time, place, event description--into an easy invite for your guests.
The second is in the final Review step of sending out your email, where you'll also see an option to add or remove the calendar invite to your email.
Just remember to double check the information in the calendar attachment if you've changed your Event Settings lately. You wouldn't want to send out an invitation for the wrong date!