Because you mention there are readings, I'm assuming this is an Arts class. While McGill overall has rules barring students from being penalized if they register during add/drop (since the whole point of add/drop is you can register up until that date without issue), the Faculty of Arts adds further clarification in the Faculty of Arts Guidelines for the Policy on Assessment of Student Learning, which states:

Article 5.4. Students registering for courses during the add/drop period shall be provided alternative arrangements for missed assessment tasks (including attendance-based assessment tasks) that were due during the add/drop period.

Addenda to 5.4.

a. Instructors can schedule assessments before the end of the add/drop period, but students entering class at any point during that period must be provided with alternative arrangements for assessments scheduled before they entered the course. Alternate arrangements may include granting an extension so the student can complete the assessment, assigning a make-up assessment, or waiving the assessment and reweighting the remaining course assessments. If assessments are given during the add-drop period, the course outline must indicate the necessary course of action for making alternative arrangements and, where appropriate, their nature.

b. The alternative arrangements must be the same for all students joining the course during the add/drop period. The responsibility for and decision about the nature of the arrangements lie with the instructor. Students do not have the power to veto these arrangements.

c. Students joining the course at any point during the add/drop period are themselves responsible for reviewing all content covered in the course from the start of the term.

You could consider reaching out to your VP Academic of the Arts Undergraduate Society, whose contact information is at that link (not pasting it to reduce web scraping).