How do you 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?

Solution 1:

Absentee can have a stronger meaning than just "not being on-site". oxforddictionaries.com gives us:

A person who is expected or required to be present at a place or event but is not

Solution 2:

You could use the term "non-attendee".

Non-attendees simply just refers to:

a person who is absent at a specified event.

Solution 3:

I think the only way you can avoid giving the impression that they should be there but are not ( as happens with absentees, non-attendees etc) is to call them non-participants.

I believe that makes clear that there was never any plan for them to be there.