In your meeting room's tools at the screen's right bottom, you'll find a "More options" (...) icon.
Clicking the icon will prompt a popup to emerge. Choose Participants - the third option from the top...
... which will invoke the Participants list overlay, listing the people who are already in attendance and the number of available spots left.
It's important to note, however, that attendees are not required to provide their real names in order to enter the meeting. Additionally, some that completely omit a name will still be admitted into the meeting but will have the default designation "Guest-X" automatically displayed on their in-meeting virtual name tag.