Why are some attendees missing from an event? Why is a specific person incorrectly listed an attendee when they shouldn't be?

If you have not synced recently, this may be because there has been a change in your SIS since your last SchoolCal sync. You will want to do a full sync now using the green button on the flow. This will visit your SIS and pull the data and then push it to your users' calendars.

If you have synced recently, you will want to check your attendee-level filters. By unselecting certain grade levels or faculty roles, such as "Other", you can limit the individuals who will be added as event attendees. You will need to save the filter settings and re-sync to see changes take effect. You can read more about attendee-level filters here.