"All visitors to the factory" is the complete subject.
Complete Predicate: must report at the office Complete Subject: All visitors to our school
all visitors to our school
All visitors to your school
all visitors
must report at the office
must report at the office
must report at the office
must report at the office
must report at the office
must report at the office
it means they either are in trouble, or they are there for a good reason. I'd just say their in trouble.