There are two ways to add attendees to a meeting request:
CC the attendees directly on the email to Clara
Include their email addresses in the body of your email
If there are multiple people CC'd on the thread, make sure to specify in your message who should be a required attendee and/or who should be optional. Clara will follow your instructions accordingly.
Do I need to CC Clara for it to schedule a meeting?
Yes. CC'ing Clara is how it knows you're requesting an action. Think of it the same way you would loop in a human assistant — add Clara to the email thread and tell it what you'd like it to do.