Until a meeting has been confirmed, you can edit the attendance in the minutes.
On the Meeting details page, there is a list of attendees and guests who were listed on the agenda.
From here, you can drag an attendee to the apologies column, add new attendees, and write any notes that are pertinent to the attendees.
Can I change a Board/Committee member in existing meetings to an attendee?
If you have made an update to your People List it will only affect future meetings, not existing ones.
For your existing meeting, you'll need to manually remove the person from the attendee list, then add then under the Guests field on your meeting page.