Learn English – How to describe someone who isn’t attending a certain event

I'm working on a software project where we need to label people depending on their presence.

Assume an event and people are attending it, we call them the attendees.

I assumed I would call those who aren't attending the event "unattending", but when I looked it up, I saw that it was basically a synonym for "careless", which isn't right at all.

I figured I could call them "absent", but in our context that could also refer to people who are just temporarily not on site.

What is the proper term to describe people who aren't planned to attend the event?