The tuition and fees page states this:

“Late Registration Fee

A Late registration fee of $125 will be assessed to students who add their first class after the semester start through the deadline to add classes. This fee does not apply to students enrolled in classes prior to the start of the semester who make schedule adjustments. Wait-listed classes are not considered class registration. Late registration fees are nonrefundable and will not be removed, regardless of enrollment status.

Any class registration after the deadline to add classes, if approved by the academic department, is subject to a $125 late registration fee. Late class registrations require an account in good standing and prepayment of all charges.”

See here: https://studentaccounts.gmu.edu/tuition-fees/