There is a new lobby setting coming this spring, that will guarantee only invited users are allowed to bypass the lobby. If an invited user should forward an invitation, the user who gets the forwarded invitation will have to wait in the lobby even if the original invite allowed the original user to join directly.
Users can find this option on the meeting option page of a meeting.
This feature was announced some time back, but the release have been slightly delayed. According to the Admin Message center, we should expect this feature soon.