Hi,
Live Meeting lobby is a virtual waiting area for the meeting participants and allows the presenter to screen participants and permit or deny entry. By sending meeting invitees to the Meeting Lobby instead of directly to the meeting, the presenter can control who enters the meeting, as well as the time that they enter.
You can enable meeting lobby in the meeting options of LiveMeeting Conferencing Administration Console. After save the meeting options, you can see the meeting lobby URL from the meeting details page. Then the users can enter the meeting lobby and you can choose to allow which users enter the meeting.
Here is a video clip which has detailed steps:
http://communityclips.officelabs.com/Video.aspx?videoId=7e7c8483-cba4-4eb2-afc2-de263780e77c
Hope this helps.
Lu Zou