What does "the meeting is full" mean?
If you see a "The meeting is full", then the session already has the maximum number of attendees that are allowed. You will not be able to join the session unless one of the attendees who has already joined leaves the session.
This is because each organizer's account has a subscription plan with a maximum number of attendees that they can have per session. Once that maximum is reached, no more attendees can join.