I have the same thing happen with my company's CEO also!
In Officely, select “view more” on the day you want to check them in. Then scroll down to “Attendees” and select “Manage Attendees”. You should then be able to select “Add people”. And when you add them in, you can choose to send them a notification that you added them or you can uncheck that and they won’t be notified. I think you do need to be an admin to add folks in