You can add anyone you want as an attendee in a meeting. Those persons do not need to be users in your ContractZen account.
When you start typing the attendee name, the service suggests all the users who have access right to that category you are in (Board, Shareholders', Executive, Other). When you add a new attendee, next time the service remembers those persons and suggests them as well. Admins can remove users from this contact list by going to Account => Edit Settings => Manage Contacts.
However, you can send an invitation and scheduling request only to people who are users in your ContractZen account. When you click Invite or Send Scheduling Request, a popup opens and by default the meeting attendees who have access rights to this category are suggested. Everyone who needs to access the meeting (and other) material in your account, needs to be users.