User selection screen 'Participants' doesn't work

The user selection screen doesn't seem to work.

For example, when I set up my BBB like this:

Other users can still join the meeting. What we want is to set up a private meeting with User 'Test 4'. This can be done through activity restriction in Moodle, but what is the use of the 'participants' interface in BBB? Teachers seem to think that they can control access and roles to the BBB using this option.