Why can't I see a meeting participant?

If there's a meeting participant you can't see under the "Attendees list", it may be because the meeting participants were invited through a distribution list and have chosen to reject the meeting.

A distribution list is a contact group in Outlook, that is used to send mails to multiple people at once.

If users are invited through a distribution list and choose to reject the meeting, they will not be able to see users in the "Attendee list". Normally, there will be a mark over the attendee's name who declined the meeting.