You could use the registration option when scheduling a meeting and make it a requirement for attendance. Another option would be to exclude the password in the meeting invite and you can send it separately. If everyone that's invited has an account on your site you can also enable the option to not let guests join the meeting. It won't stop internal users from joining if the lobby is unlocked. Definitely use the lock room immediately and then you can just admit those people that you want in the meeting. Hopefully one of these options will work for you.
