Can not add invitees to a calendar event after the event has been created

I can not find a way to add an invitee after the event has been created and the invite emails sent. Is it possible?

Below is the already created event, where I do not see the field to "add invitees" any more:

Here is an event that has not yet been created, thus possible to add invitees:

EDIT: I use Exchange as the backend for the calendar... though had same issues with Google Calendar as backend as well.

Using Calendar Version 9.0 (2155.15)

I solved the issue of not being able to add an invitee by exporting all my appointments and creating a new iCloud Calendar.

• go to File > New Calendar > iCloud
• create a new calendar
• go to File > Export > Export...
• export the old calendar
• go to File > Import...
• select the file to which you exported the old calendar
• click Import
• select the newly added calendar
• click Ok
• delete the old calendar

If you click on the list of invitees, there should be an "Add Invitees" field which appears below the list.