When a session is full attendees are no longer able to register for it. However, you are able to enable a waitlist which allows attendees to indicate their interest in a full session.
Enable a waiting list
On the Scheduling tab select the Confirmed Attendees menu item. Locate the session you want to enable a waitlist for and click the "Attendees" button. On the right-hand side you are able to toggle to either enable or disable the waiting list.
When the waitlist is toggled to be enabled you are able to enter the number of waitlist spaces you want available. Don't forget to click the "Save" button.
Note: As spaces become available attendees are automatically moved from the waiting list to the registered list on a "first come, first served" basis.