About
If you want to invite a large amount people to a Google Calendar event, you can simply invite a Google Group to an event. This will extend the invitation for the event for each person that is part of the Google Group.
For instructions on how to create a Google Group, please see this KBA: Creating a Google Group.
If you have to create a Google Group with a large amount of people, please see this KBA: Adding a Large Amount of People to a Google Group.
Environment
Google Groups
Google Calendar
Steps
Inviting a Google Group to a Google Calendar Event can be done the same way that you invite guests in general. When editing the event, type the name of the Google Group into the Guests field. Once you click on the name of the Google Group, anyone that is part of the Group will receive an invitation. You can verify this by clicking on the name of the Google Group, which will drop down with the list of people included.