Salesforce Classic:

Event invitee email notifications are only sent from Salesforce when you Invite People to an Event and Schedule Resources in Salesforce Classic.

Salesforce Lightning Experience:

It is not possible to send Event attendees email notifications in Lightning without setting up and configuring Einstein Activity Capture (EAC) for your organization.

Help Article – https://help.salesforce.com/articleView?id=000323375&type=1&mode=1

1. Create Event record in Salesforce classic.

2. Click Add Attendees to add Contacts, Users and Leads.

3. Click Save and Update button.

Organizer Email Notification:

Attendee Email Notification: