Your personal Google Calendar ID is your email address. If someone wants to invite your personal calendar they'll enter your email address (e.g. jdoe@ingeniumschools.org).
Your Departmental / Site Google Calendar ID can be found by clicking on the 3 dots next to your department calendar
Then click on SETTINGS and SHARING and scroll down to the section labelled INTEGRATE CALENDAR. The Calendar ID is the email address you use to invite a departmental calendar.