You must set up the actual appointment slots before others can book them.

Enter their username in the 'Add Guests' field (to the right of the 'Title' field).

a professor might add a TA to appointments for office hours, but would NOT add a student). When adding someone to the appointments, s/he will be invited to every appointment so only add people who will be assisting with all the appointments (i.e. There are several things you can do with appointment slots: Note: Appointment Slots are currently only available to Google Apps users (i.e., not to personal Gmail users).