To expand on what Fred said, the controls around who can join a meeting, passwords, scheduling, or any other business logic decisions exist outside of the core of BigBlueButton. Different front-ends, LMSs, or Greenlight can implement all of that functionality differently and to the needs of the front-end. What you are asking for might be provided in an existing front-end or it might require writing a new one to support your needs. I'm not aware of any front-ends to BBB that do what you're asking for.