Microsoft Outlook allows me to turn off "Allow Forwarding" on a meeting invite.
The invites sent out by Absorb LMS are forwardable, allowing learners to add class sessions to their calendar (along with any details included in the meeting (location, online meeting URLs, etc.)) without enrolling in the session. This short-circuits the enrollment process and any "max attendees" limit you may have put on the course session.
As a default, meeting invites should have this disabled with an option to turn it on.
Related, but possibly separate - the meeting URL and meeting details are available to learners without enrolling in the course. We should have the option of hiding those details from learners until they are enrolled in a given session.