Once all your attendees are in the meeting you can lock down the meeting so no other attendee can join.
To lock the meeting, click on Manage Participants on the meeting bar.
This will open the participant bar. Please click on the "More" options drop-down box. In here you will will see some options listed. Please click on "Lock Meeting"
You will get a pop-up saying users will no longer be allowed in, click "Yes" to confirm the locking of the meeting
and any new user will get the following message:
