This functionality applies to the Employment module. If you do not have the Employment module, to add a substitute teacher to a class so they have access to the class in the portal, you need to add the substitute teacher to the “Permissions” tab of the class record, then remove the permission when they are done.
Navigate to the faculty member's attendance detail record for the date that they will be out. (Run a Find Staff/Faculty Master Attendance query to do so). On this record, turn on the Substitute Required flag and add the substitute teacher's name to the Substitute field if they will be covering for all of the teacher's classes that day. If it's only a couple of classes, select a substitute in the input grid and then click Update.