By default, will work with everyone on the email chain to confirm a time and assume they are required for the meeting. Often times not everyone CC'd in on an email thread is required to attend the meeting, so you always have the option to update who is optional when editing a meeting.
You can also tell Scheduler explicitly who is optional by listing it out, i.e. Scheduler, set up a /zoom next week. Brian and Kim are optional.

You can update the status of the guests at any time by going to the Meeting Page (simply search for the meeting by title or guest). Click on "Edit Invite" and then in the Guest panel on the right hand side you can select the silhouette icon to change the status. A filled icon indicates a required guest, and an outlined icon indicates an optional guest.

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