One of my teammates is in a different timezone and isn't a Clockwise user. Can I mark a meeting with them as flexible?

Yes. Clockwise can automatically tell what timezone your teammates are located in using their work calendar settings. Clockwise will take into account that non-user's default working hours (Clockwise infers meeting hours of 10:00 am - 4:00 pm Monday-Thursday, and 10:00 am - 3:30 pm on Friday) to reflect their local timezone.