RingCentral Meetings - Waiting Room | Host Controls
Enable Waiting Room account-wide
1. Go to Tools > Meetings > Meetings Settings.
2. Scroll down to In Meeting (Advanced) and enable Waiting room.

Enable Waiting Room Extension Settings
1. Go to Settings > Meetings.
2. Scroll down to In Meeting (Advanced) and enable Waiting room.

How it works when enabled
If Waiting Room is enabled, Enable join before host option is automatically disabled. Manage Participants icon will illuminate when a new Participant joins the meeting. The Host can click Admit to let in the new Participant immediately or click See waiting room to open the Manage Participants window. The Host can hover on the Participant's name to Admit or Remove.

See Also:
RingCentral Meetings - Desktop | Host Controls