If the meeting was created before someone has been added to BoardPro, they will not automatically appear in the meeting attendees list - here's how to add them.
Check that they have been added to BoardPro: Go into the People List at the top of the page.
If they are not on the list you can add them. See Adding People into BoardPro
Add then to the list of attendees: Once added to the People List, you can now add them as an attendee to the meeting.
On the Meeting Details card, you will see a box "Add from People's List": Select the name you need to add. They will now appear on the agenda.
If everyone has been added from your People List a box will open with the options to add a person as a guest to the agenda or add them to your organisation.