You can decide if you'd like invitees to add guests as they go through the booking flow. An invitee can add up to 10 guest email addresses when booking a one-on-one event type. Their guests will be sent a notification about the upcoming meeting. You can view any added guests from the event in your Scheduled Events tab. 

Note: This feature is not available for group event types, since you need to control the number of invitees who can attend a group event at a given time. 
By default, the additional guest feature is turned on. To turn off the ability to add guests:

1. From your event type, select the Invitee Questions section.
2. Hover over the Name fields and select the pencil icon.
3. Deselect Allow invitees to add additional guests.
4. Select Apply.
5. Select Save & Close.

Is there a way to turn off the global default for this? I get I can turn it off for every individual event type, but I would prefer to opt in to it than to opt out of it in my industry (higher ed). 

Hey @Holly45420 - great question!

At this time, there is no way to turn off the default “add additional guests” setting for one-on-one event types. I am truly sorry for the inconvenience! You will have to go into each one-on-one and turn this off if you do not wish to allow invitees to add additional guests, at this time.

I will make sure to include your feedback in our monthly report for the product team! Have a great day. =)