In a webinar there are two kinds of people:
Panelists (the host, co-hosts and speakers) are on stage. They can use their microphone, camera and screen share.
Attendees watch and listen. They can't turn on a mic or camera or share their screen. They can chat, ask questions in Q&A, raise their hand and react. Attendees see how many people are watching, but not the list of attendees.
Audience count The top bar shows "Webinar · N watching". In People, the host sees Panelists and Attendees listed separately.
Bring an attendee on stage The host or a co-host opens People, finds the attendee and clicks Promote to panelist. The attendee is asked "… invited you to speak" and chooses Join as panelist or Stay attendee. The invitation expires after 2 minutes. A new panelist joins muted with the camera off.
To send a panelist back, choose Move to attendees from their menu in People.