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, Amy, and Andrew explicitly who in the meeting they should work by listing out the participants. This indicates that they should only schedule with those listed and all others (on the To or CC field) will be made optional automatically.

Additionally, you can also set a guest to be optional on your meetings page on Just click into the meeting on your dashboard and then click on the edit icon next to your guest’s name. There you can set a guest to be required or optional with a click.

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