See also the official documentation:
Help Center > Members Manual > Adding events from outside calendar invitations

Adding calendar events by email

Users of Premium (paid for) groups can add events to a group calendar by email. The basic process is:

• In your email client, create a new event.
• Enter your event details.
• Set the event recurrence details (if needed).
• Add your group email address as an attendee.
• Send the event invitation to your group.

Here are some sample screenshots of the process using Outlook 2016. Other email clients will vary but the process will be the same.

1 - Choose the date on the calendar and click New Appointment.

2 - Enter the event details.

3 - Set the Recurrence if needed.

4 - Click Invite Attendees.

5 - Enter your group email address.

6 - Send the invite.

A message will be sent to your group with the calendar details in an attachment (this may require moderation depending upon your group settings).

When the message is processed, the event will be added to your group calendar:

Note: There is currently a limit to how far in advance you can add a calendar event by email and that is set to 1 year.