We prefer an opt-in mode, instead of opt-out mode. Is this possible?

Yes, that is possible. If you prefer not to provide an attendee list or the attendees who previously did not have profiles now want to be listed, they do have the option to “opt-in” and build their profiles themselves. This requires you to provide the event name and invitation code (if you are using an invitation code) to your attendees so that they can find your event.